runtime: fix crash when returning from syscall during gc
runtime: fix crash when returning from syscall during gc

gp->m can go from non-nil to nil when it re-enters schedule().

R=golang-dev
CC=golang-dev, rsc
http://codereview.appspot.com/5245042

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go