runtime: initialize complete Hiter during mapiterinit

The garbage collector will scan these pointers,
so make sure they are initialized.

LGTM=bradfitz, khr
R=khr, bradfitz
CC=golang-codereviews
https://golang.org/cl/80960047

src/pkg/runtime/hashmap.goc

@@ -1004,6 +1004,14 @@ func reflect·mapdelete(t *MapType, h *Hmap, key *byte) {
 
 #pragma textflag NOSPLIT
 func mapiterinit(t *MapType, h *Hmap, it *Hiter) {
+	// Clear pointer fields so garbage collector does not complain.
+	it->key = nil;
+	it->value = nil;
+	it->t = nil;
+	it->h = nil;
+	it->buckets = nil;
+	it->bptr = nil;
+
 	if(h == nil || h->count == 0) {
 		it->key = nil;
 		return;