runtime: restore correct m in gtraceback

If gtraceback is used to get a stack trace of a g running in the same m,
as can happen if we collect a stack trace from a g0, then restore the
old m value, don't clear it.

Change-Id: I950a3c182d0b79dd84204c59530c57edb1e100e1
Reviewed-by: Than McIntosh <>
1 file changed