Android background crash in libkb.GlobalContext.Trace

I saw this crash in my adb logcat output today:

```
08-17 15:02:31.837 5276 5276 E oid.apps.photo: Not starting debugger since process cannot load the jdwp agent.
08-17 15:02:31.889 5299 5299 E ndroid.contact: Not starting debugger since process cannot load the jdwp agent.
08-17 15:02:31.896 5287 5287 E id.partnersetu: Not starting debugger since process cannot load the jdwp agent.
08-17 15:02:31.897 5294 5294 E ybase.ossifrag: Not starting debugger since process cannot load the jdwp agent.
08-17 15:02:31.926 5315 5315 E le.android.kee: Not starting debugger since process cannot load the jdwp agent.
08-17 15:02:32.366 5299 5401 E ndroid.contact: The String#value field is not present on Android versions >= 6.0
08-17 15:02:32.799 5294 0 E Go : panic: runtime error: invalid memory address or nil pointer dereference
08-17 15:02:32.799 5294 0 E Go : [signal SIGSEGV: segmentation violation code=0x1 addr=0x4 pc=0xd5d88c10]
08-17 15:02:32.799 5294 0 E Go :
08-17 15:02:32.799 5294 0 E Go : goroutine 17 [running, locked to thread]:
08-17 15:02:32.799 5294 0 E Go : github.com/keybase/client/go/libkb.(*GlobalContext).Trace(0x0, 0xd668b604, 0xe, 0xd74b0db8, 0x9173a03c)
08-17 15:02:32.799 5294 0 E Go : /tmp/go-android/src/github.com/keybase/client/go/libkb/util.go:509 +0x14
08-17 15:02:32.799 5294 0 E Go : github.com/keybase/client/go/bind.BackgroundSync()
08-17 15:02:32.799 5294 0 E Go : /tmp/go-android/src/github.com/keybase/client/go/bind/keybase.go:354 +0x44
08-17 15:02:32.799 5294 0 E Go : _/var/folders/4g/mh1d9tz91d3f9p5p716dkdd40000gn/T/gomobile-work-849699548/gomobile_bind.proxykeybase__BackgroundSync()
08-17 15:02:32.799 5294 0 E Go : /var/folders/4g/mh1d9tz91d3f9p5p716dkdd40000gn/T/gomobile-work-849699548/gomobile_bind/go_keybasemain.go:326 +0x14
08-17 15:02:32.799 5294 0 E Go : _/var/folders/4g/mh1d9tz91d3f9p5p716dkdd40000gn/T/gomobile-work-849699548/gomobile_bind._cgoexpwrap_b7f20329c467_proxykeybase__BackgroundSync()
08-17 15:02:32.799 5294 0 E Go : _/var/folders/4g/mh1d9tz91d3f9p5p716dkdd40000gn/T/gomobile-work-849699548/gomobile_bind/_obj/_cgo_gotypes.go:506 +0x14
```

I'm running 2.5.0-20180815151913+8eb2dbfe76 according to the "About" screen.

Start with go/libkb/util.go at line 509 and go/bind/keybase.go at line 354, following the BackgroundSync call shown in the Android stack trace. Reproduce or inspect the background path using the supplied adb logcat output and verify that it no longer reaches GlobalContext.Trace with a nil receiver.

Written by the indexing model from the issue text.