runtime: don't crash if panicstring called with no m
runtime: don't crash if panicstring called with no m

It's possible for runtime_panicstring to be called with no m if a
signal handler, or scheduler innards, do something wrong.  If that
happens carry on with the panic rather than crashing.

Change-Id: I8ad04f1f9d7655b4a4e9a983f439f2cc3ac6b269
Reviewed-on: https://go-review.googlesource.com/41137
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend