runtime: reset typed dangling pointer

If for whatever reason seh points into Go heap region,
the dangling pointer will cause memory corruption during GC.
Update #5193.

R=golang-dev, alex.brainman, iant
CC=golang-dev
https://golang.org/cl/8402045

src/pkg/runtime/proc.c

@@ -695,6 +695,7 @@ runtime·dropm(void)
 
 	// Undo whatever initialization minit did during needm.
 	runtime·unminit();
+	m->seh = nil;  // reset dangling typed pointer
 
 	// Clear m and g, and return m to the extra list.
 	// After the call to setmg we can only call nosplit functions.