runtime/pprof: nil pointer panic in TestMathBigDivide and android/arm

[eliasnaur]

The Android/arm builder failed once:

https://build.golang.org/log/49d8e3585a3501d3f82d92c762aa3d5724d45198

with the stack trace:

--- FAIL: TestMathBigDivide (0.31s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xf738b874]

goroutine 53 [running]:
testing.tRunner.func1(0xd7038120)
	/private/var/folders/f6/d2bhfqss2716nxm8gkv1fmb80000gn/T/workdir-host-darwin-amd64-eliasnaur-android/go/src/testing/testing.go:712 +0x22c
panic(0xf740cef8, 0xf74e1418)
	/private/var/folders/f6/d2bhfqss2716nxm8gkv1fmb80000gn/T/workdir-host-darwin-amd64-eliasnaur-android/go/src/runtime/panic.go:489 +0x218
runtime/pprof.fprintStack(0xf74e4e68, 0xd700c000, 0xd74700f0, 0x2, 0x2)
	/private/var/folders/f6/d2bhfqss2716nxm8gkv1fmb80000gn/T/workdir-host-darwin-amd64-eliasnaur-android/go/src/runtime/pprof/pprof_test.go:340 +0x54
runtime/pprof.profileOk.func1(0x1, 0xd74700f0, 0x2, 0x2, 0x0)
	/private/var/folders/f6/d2bhfqss2716nxm8gkv1fmb80000gn/T/workdir-host-darwin-amd64-eliasnaur-android/go/src/runtime/pprof/pprof_test.go:183 +0xd4
runtime/pprof.parseProfile(0xd7038120, 0xd7002d00, 0x719, 0x890, 0xd6fb5ea4)
	/private/var/folders/f6/d2bhfqss2716nxm8gkv1fmb80000gn/T/workdir-host-darwin-amd64-eliasnaur-android/go/src/runtime/pprof/pprof_test.go:97 +0x130
runtime/pprof.profileOk(0xd7038120, 0x0, 0x0, 0x0, 0xd7002d00, 0x719, 0x890, 0x0, 0x0, 0x88b1f, ...)
	/private/var/folders/f6/d2bhfqss2716nxm8gkv1fmb80000gn/T/workdir-host-darwin-amd64-eliasnaur-android/go/src/runtime/pprof/pprof_test.go:181 +0x110
runtime/pprof.testCPUProfile(0xd7038120, 0x0, 0x0, 0x0, 0xf742a140)
	/private/var/folders/f6/d2bhfqss2716nxm8gkv1fmb80000gn/T/workdir-host-darwin-amd64-eliasnaur-android/go/src/runtime/pprof/pprof_test.go:140 +0x11c
runtime/pprof.TestMathBigDivide(0xd7038120)
	/private/var/folders/f6/d2bhfqss2716nxm8gkv1fmb80000gn/T/workdir-host-darwin-amd64-eliasnaur-android/go/src/runtime/pprof/pprof_test.go:349 +0x3c
testing.tRunner(0xd7038120, 0xf742a144)
	/private/var/folders/f6/d2bhfqss2716nxm8gkv1fmb80000gn/T/workdir-host-darwin-amd64-eliasnaur-android/go/src/testing/testing.go:747 +0xbc
created by testing.(*T).Run
	/private/var/folders/f6/d2bhfqss2716nxm8gkv1fmb80000gn/T/workdir-host-darwin-amd64-eliasnaur-android/go/src/testing/testing.go:789 +0x288
exitcode=2go_android_exec: adb shell rm -rf /data/local/tmp/pprof.test-31327
FAIL	runtime/pprof	5.818s

I'm filing this issue so that we don't forget and in case someone else have an idea why it happens.