Currently, if lock or unlock calls throw because the g.m.lock count is
corrupted, we're unlikely to get a stack trace because startpanic_m
will itself attempt to acquire a lock, causing a recursive failure.

Avoid this by forcing the g.m.locks count to a sane value if it's
currently bad.

This might be enough to get a stack trace from #25128.

Change-Id: I52d7bd4717ffae94a821f4249585f3eb6cd5aa41
Reviewed-on: https://go-review.googlesource.com/120416Reviewed-by: Ian Lance Taylor <iant@golang.org>