runtime: reset dangling typed pointer

+untype it because it can point to different types
Update #5193.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/8454043

src/pkg/runtime/proc.c

@@ -1178,6 +1178,7 @@ park0(G *gp)
 	if(m->waitunlockf) {
 		m->waitunlockf(m->waitlock);
 		m->waitunlockf = nil;
+		m->waitlock = nil;
 	}
 	if(m->lockedg) {
 		stoplockedm();

src/pkg/runtime/runtime.h

@@ -317,7 +317,7 @@ struct	M
 	bool	needextram;
 	void*	racepc;
 	void	(*waitunlockf)(Lock*);
-	Lock*	waitlock;
+	void*	waitlock;
 	uint32	moreframesize_minalloc;
 
 	uintptr	settype_buf[1024];