cli: span use after finish crash

[andreimatei]

Sentry reports of the following. Must be logging with a context who's span has already been Finish()ed.

*log.safeError: panic.go:28: runtime.errorString: runtime error: index out of range
  File "github.com/cockroachdb/cockroach/vendor/golang.org/x/net/trace/trace.go", line 772, in addEvent
  File "github.com/cockroachdb/cockroach/vendor/golang.org/x/net/trace/trace.go", line 801, in LazyPrintf
  File "github.com/cockroachdb/cockroach/pkg/util/tracing/tracer_span.go", line 390, in LogFields
  File "github.com/cockroachdb/cockroach/pkg/util/log/trace.go", line 131, in eventInternal
  File "github.com/cockroachdb/cockroach/pkg/util/log/structured.go", line 153, in addStructured
  File "github.com/cockroachdb/cockroach/pkg/util/log/log.go", line 55, in logDepth
  File "github.com/cockroachdb/cockroach/pkg/util/log/log.go", line 61, in Shout
  File "github.com/cockroachdb/cockroach/pkg/cli/start.go", line 767, in runStart
  File "github.com/cockroachdb/cockroach/pkg/cli/error.go", line 39, in func1
  File "github.com/cockroachdb/cockroach/pkg/cli/error.go", line 101, in func1
  File "github.com/cockroachdb/cockroach/vendor/github.com/spf13/cobra/command.go", line 698, in execute
  File "github.com/cockroachdb/cockroach/vendor/github.com/spf13/cobra/command.go", line 783, in ExecuteC
  File "github.com/cockroachdb/cockroach/vendor/github.com/spf13/cobra/command.go", line 736, in Execute
  File "github.com/cockroachdb/cockroach/pkg/cli/cli.go", line 176, in Run
  File "github.com/cockroachdb/cockroach/pkg/cli/cli.go", line 54, in Main
  File "github.com/cockroachdb/cockroach/pkg/cmd/cockroach/main.go", line 29, in main
  File "runtime/proc.go", line 198, in main

In fact I think I see the problem. Fix incoming.

[andreimatei]

Sentry issue: COCKROACHDB-H2

[andreimatei]

Sentry issue: COCKROACHDB-G1