runtime: add check for malloc in a signal handler

Change-Id: Ic8ebbe81eb788626c01bfab238d54236e6e5ef2b
Reviewed-on: https://go-review.googlesource.com/9964
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/malloc.go

@@ -509,6 +509,9 @@ func mallocgc(size uintptr, typ *_type, flags uint32) unsafe.Pointer {
 	if mp.mallocing != 0 {
 		throw("malloc deadlock")
 	}
+	if mp.gsignal == getg() {
+		throw("malloc during signal")
+	}
 	mp.mallocing = 1
 
 	shouldhelpgc := false