Using a transform function to/from an interface cause a nil pointer dereference

[Stebalien]

At least when used with CBOR tags.

Repro: master...Stebalien:repro/nil-deref

Trace:

  * /home/steb/projects/go/src/github.com/polydawn/refmt/obj/objFixtures_test.go 
  Line 1540: - runtime error: invalid memory address or nil pointer dereference 
  goroutine 6 [running]:
  panic(0x5d2260, 0x78f230)
  	/usr/lib/go/src/runtime/panic.go:513 +0x1b9
  github.com/polydawn/refmt/obj.(*unmarshalMachineWildcard).Step(0xc00033c058, 0xc0000e9c80, 0xc0000e9c80, 0xc0002ce9a0, 0x5fdc60, 0xc00047ed01, 0xc0002ce9a0)
  	/home/steb/projects/go/src/github.com/polydawn/refmt/obj/unmarshalWildcard.go:32 +0x39
  github.com/polydawn/refmt/obj.(*Unmarshaller).Step(0xc0000e9c80, 0xc0002ce9a0, 0x2, 0x2, 0x1)
  	/home/steb/projects/go/src/github.com/polydawn/refmt/obj/unmarshal.go:60 +0x4c
  github.com/polydawn/refmt/obj.TestUnmarshaller.func1.1.1.2()
  	/home/steb/projects/go/src/github.com/polydawn/refmt/obj/objFixtures_test.go:1540 +0x1d1
  github.com/jtolds/gls.(*ContextManager).SetValues.func1(0x0)
  	/home/steb/projects/go/src/github.com/jtolds/gls/context.go:97 +0x3f2
  github.com/jtolds/gls.EnsureGoroutineId(0xc000487500)
  	/home/steb/projects/go/src/github.com/jtolds/gls/gid.go:19 +0x103
  github.com/jtolds/gls.(*ContextManager).SetValues(0xc00004a550, 0xc0004874a0, 0xc0002c2e20)
  	/home/steb/projects/go/src/github.com/jtolds/gls/context.go:63 +0x147
  github.com/polydawn/refmt/obj.TestUnmarshaller.func1.1.1()
  	/home/steb/projects/go/src/github.com/polydawn/refmt/obj/objFixtures_test.go:1532 +0x2b3
  github.com/jtolds/gls.(*ContextManager).SetValues.func1(0x0)
  	/home/steb/projects/go/src/github.com/jtolds/gls/context.go:97 +0x3f2
  github.com/jtolds/gls.EnsureGoroutineId(0xc0004873e0)
  	/home/steb/projects/go/src/github.com/jtolds/gls/gid.go:19 +0x103
  github.com/jtolds/gls.(*ContextManager).SetValues(0xc00004a550, 0xc000487380, 0xc0002c2de0)
  	/home/steb/projects/go/src/github.com/jtolds/gls/context.go:63 +0x147
  github.com/polydawn/refmt/obj.TestUnmarshaller.func1.1()
  	/home/steb/projects/go/src/github.com/polydawn/refmt/obj/objFixtures_test.go:1520 +0x3bd
  github.com/jtolds/gls.(*ContextManager).SetValues.func1(0x0)
  	/home/steb/projects/go/src/github.com/jtolds/gls/context.go:97 +0x3f2
  github.com/jtolds/gls.EnsureGoroutineId(0xc0004872c0)
  	/home/steb/projects/go/src/github.com/jtolds/gls/gid.go:19 +0x103
  github.com/jtolds/gls.(*ContextManager).SetValues(0xc00004a550, 0xc000487260, 0xc0002c2d60)
  	/home/steb/projects/go/src/github.com/jtolds/gls/context.go:63 +0x147
  github.com/polydawn/refmt/obj.TestUnmarshaller.func1()
  	/home/steb/projects/go/src/github.com/polydawn/refmt/obj/objFixtures_test.go:1511 +0x20d
  github.com/jtolds/gls.(*ContextManager).SetValues.func1(0x0)
  	/home/steb/projects/go/src/github.com/jtolds/gls/context.go:97 +0x3f2
  github.com/jtolds/gls.EnsureGoroutineId.func1()
  	/home/steb/projects/go/src/github.com/jtolds/gls/gid.go:24 +0x2e
  github.com/jtolds/gls._m(0x0, 0xc0003b85c0)
  	/home/steb/projects/go/src/github.com/jtolds/gls/stack_tags.go:74 +0x31
  github.com/jtolds/gls.github_com_jtolds_gls_markS(0x0, 0xc0003b85c0)
  	/home/steb/projects/go/src/github.com/jtolds/gls/stack_tags.go:54 +0x35
  github.com/jtolds/gls.addStackTag(0x0, 0xc0003b85c0)
  	/home/steb/projects/go/src/github.com/jtolds/gls/stack_tags.go:49 +0x3a
  github.com/jtolds/gls.EnsureGoroutineId(0xc00039f410)
  	/home/steb/projects/go/src/github.com/jtolds/gls/gid.go:24 +0xc3
  github.com/jtolds/gls.(*ContextManager).SetValues(0xc00004a550, 0xc00039f3b0, 0xc0003b8580)
  	/home/steb/projects/go/src/github.com/jtolds/gls/context.go:63 +0x147
  github.com/polydawn/refmt/obj.TestUnmarshaller(0xc0000c6200)
  	/home/steb/projects/go/src/github.com/polydawn/refmt/obj/objFixtures_test.go:1509 +0x99
  testing.tRunner(0xc0000c6200, 0x616c50)
  	/usr/lib/go/src/testing/testing.go:827 +0xbf
  created by testing.(*T).Run
  	/usr/lib/go/src/testing/testing.go:878 +0x353