runtime: zero d.free field

Not programming in Go anymore:
have to clear fields in local variables.

R=ken2
CC=golang-dev
https://golang.org/cl/7002053

src/pkg/runtime/cgocall.c

@@ -132,6 +132,7 @@ runtime·cgocall(void (*fn)(void*), void *arg)
 		d.link = g->defer;
 		d.argp = (void*)-1;  // unused because unlockm never recovers
 		d.special = true;
+		d.free = false;
 		g->defer = &d;
 	}
 
@@ -237,6 +238,7 @@ runtime·cgocallbackg(void (*fn)(void), void *arg, uintptr argsize)
 	d.link = g->defer;
 	d.argp = (void*)-1;  // unused because unwindm never recovers
 	d.special = true;
+	d.free = false;
 	g->defer = &d;
 
 	if(raceenabled)