Fix BranchSimpleIterator panic on close after failed SeekGE

[nopcoder]

While listing branches error from the underlying kv can cause a panic in the iterator Close.
The following occur while the iterator SeekGE failed, caused the following iterator Close to try access nil pointer.

2023/03/18 03:45:38 http: panic serving [::1]:59624: runtime error: invalid memory address or nil pointer dereference
goroutine 521 [running]:
net/http.(*conn).serve.func1()
        /Users/barak/sdk/go1.19.3/src/net/http/server.go:1850 +0x118
panic({0x107acbda0, 0x1089c4110})
        /Users/barak/sdk/go1.19.3/src/runtime/panic.go:890 +0x26c
github.com/treeverse/lakefs/pkg/kv.(*PrimaryIterator).Close(0x0)
        /Users/barak/work/lakeFS/pkg/kv/iterators.go:118 +0x1c
github.com/treeverse/lakefs/pkg/graveler/ref.(*BranchSimpleIterator).Close(0x14000949590)
        /Users/barak/work/lakeFS/pkg/graveler/ref/branch_iterator.go:96 +0x74
github.com/treeverse/lakefs/pkg/catalog.(*Catalog).ListBranches(0x1400070e840, {0x107dac7d0, 0x1400095d260}, {0x14000962069, 0x4}, {0x0, 0x0}, 0x3, {0x1400096207d, 0x4})
        /Users/barak/work/lakeFS/pkg/catalog/catalog.go:562 +0x718
github.com/treeverse/lakefs/pkg/api.(*Controller).ListBranches(0x1400c6cee00, {0x107daaab0, 0x1400094b590}, 0x1400df02600, {0x14000962069, 0x4}, {0x0, 0x140005abbc0, 0x1400095a750})
        /Users/barak/work/lakeFS/pkg/api/controller.go:1740 +0x1a0
github.com/treeverse/lakefs/pkg/api.(*ServerInterfaceWrapper).ListBranches.func1({0x107daaab0, 0x1400094b590}, 0x1400df02600)
        /Users/barak/work/lakeFS/pkg/api/lakefs.gen.go:19608 +0x7c
github.com/treeverse/lakefs/pkg/api.(*ServerInterfaceWrapper).ListBranches(0x1400c9a97a0, {0x107daaab0, 0x1400094b590}, 0x1400df02500)
        /Users/barak/work/lakeFS/pkg/api/lakefs.gen.go:19615 +0xb5c
net/http.HandlerFunc.ServeHTTP(0x1400d00a960, {0x107daaab0, 0x1400094b590}, 0x1400df02500)
        /Users/barak/sdk/go1.19.3/src/net/http/server.go:2109 +0x44
github.com/treeverse/lakefs/pkg/api.MetricsMiddleware.func1.1({0x11073c3e0, 0x1400095d140}, 0x1400df02500)
        /Users/barak/work/lakeFS/pkg/api/metrics_middleware.go:27 +0xe4
net/http.HandlerFunc.ServeHTTP(0x1400d03c0c0, {0x11073c3e0, 0x1400095d140}, 0x1400df02500)
        /Users/barak/sdk/go1.19.3/src/net/http/server.go:2109 +0x44
github.com/prometheus/client_golang/prometheus/promhttp.InstrumentHandlerCounter.func1({0x107daaae0, 0x140005ac380}, 0x1400df02500)
        /Users/barak/go/pkg/mod/github.com/prometheus/client_golang@v1.14.0/prometheus/promhttp/instrument_server.go:146 +0xc0
net/http.HandlerFunc.ServeHTTP(0x1400d03c270, {0x107daaae0, 0x140005ac380}, 0x1400df02500)
        /Users/barak/sdk/go1.19.3/src/net/http/server.go:2109 +0x44
github.com/treeverse/lakefs/pkg/api.AuthMiddleware.func1.1({0x107daaae0, 0x140005ac380}, 0x1400df02500)
        /Users/barak/work/lakeFS/pkg/api/auth_middleware.go:72 +0x3e0
net/http.HandlerFunc.ServeHTTP(0x1400cb15280, {0x107daaae0, 0x140005ac380}, 0x1400df02400)
        /Users/barak/sdk/go1.19.3/src/net/http/server.go:2109 +0x44
github.com/treeverse/lakefs/pkg/httputil.DefaultLoggingMiddleware.func1.1({0x107dab2c0, 0x14000944700}, 0x1400df02400)
        /Users/barak/work/lakeFS/pkg/httputil/logging.go:87 +0x534
net/http.HandlerFunc.ServeHTTP(0x1400d03be00, {0x107dab2c0, 0x14000944700}, 0x1400df02200)
        /Users/barak/sdk/go1.19.3/src/net/http/server.go:2109 +0x44
github.com/treeverse/lakefs/pkg/api.OapiRequestValidatorWithOptions.func1.1({0x107dab2c0, 0x14000944700}, 0x1400df02200)
        /Users/barak/work/lakeFS/pkg/api/serve.go:171 +0x2e8
net/http.HandlerFunc.ServeHTTP(0x1400d03c2a0, {0x107dab2c0, 0x14000944700}, 0x1400df02100)
        /Users/barak/sdk/go1.19.3/src/net/http/server.go:2109 +0x44
github.com/go-chi/chi/v5.(*ChainHandler).ServeHTTP(0x1400d03be40, {0x107dab2c0, 0x14000944700}, 0x1400df02100)
        /Users/barak/go/pkg/mod/github.com/go-chi/chi/v5@v5.0.0/chain.go:31 +0x48
github.com/go-chi/chi/v5.(*Mux).routeHTTP(0x140001152c0, {0x107dab2c0, 0x14000944700}, 0x1400df02100)
        /Users/barak/go/pkg/mod/github.com/go-chi/chi/v5@v5.0.0/mux.go:436 +0x22c
net/http.HandlerFunc.ServeHTTP(0x1400c999f40, {0x107dab2c0, 0x14000944700}, 0x1400df02100)
        /Users/barak/sdk/go1.19.3/src/net/http/server.go:2109 +0x44
github.com/go-chi/chi/v5.(*Mux).ServeHTTP(0x140001152c0, {0x107dab2c0, 0x14000944700}, 0x1400df02100)
        /Users/barak/go/pkg/mod/github.com/go-chi/chi/v5@v5.0.0/mux.go:87 +0x250
github.com/treeverse/lakefs/cmd/lakefs/cmd.glob..func8.3({0x107dab2c0, 0x14000944700}, 0x1400df02000)
        /Users/barak/work/lakeFS/cmd/lakefs/cmd/run.go:298 +0x144
net/http.HandlerFunc.ServeHTTP(0x1400de08840, {0x107dab2c0, 0x14000944700}, 0x1400df02000)
        /Users/barak/sdk/go1.19.3/src/net/http/server.go:2109 +0x44
net/http.serverHandler.ServeHTTP({0x1400014e960}, {0x107dab2c0, 0x14000944700}, 0x1400df02000)
        /Users/barak/sdk/go1.19.3/src/net/http/server.go:2947 +0x41c
net/http.(*conn).serve(0x140005f8000, {0x107dac728, 0x14000113a00})
        /Users/barak/sdk/go1.19.3/src/net/http/server.go:1991 +0x14e0
created by net/http.(*Server).Serve
        /Users/barak/sdk/go1.19.3/src/net/http/server.go:3102 +0x7e4

Fix #5517

[N-o-Z]

LGTM