runtime: avoid fault in heapBitsBulkBarrier

Change-Id: I0512e461de1f25cb2a1cb7f23e7a77d00700667c
Reviewed-on: https://go-review.googlesource.com/10803Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mbitmap.go

@@ -388,7 +388,7 @@ func heapBitsBulkBarrier(p, size uintptr) {
 		// unwinding the stack barriers between the current SP
 		// and p's frame.
 		gp := getg().m.curg
-		if gp.stack.lo <= p && p < gp.stack.hi {
+		if gp != nil && gp.stack.lo <= p && p < gp.stack.hi {
 			// Run on the system stack to give it more
 			// stack space.
 			systemstack(func() {