cmd/gc: Reset haspointers computation. When converting from a

slice type to an array type, the haspointer-ness may change.
Before this change, we'd sometimes get types like [1]int marked
as having pointers.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13189044

src/cmd/gc/sinit.c

@@ -686,6 +686,7 @@ slicelit(int ctxt, Node *n, Node *var, NodeList **init)
 	t->bound = mpgetfix(n->right->val.u.xval);
 	t->width = 0;
 	t->sym = nil;
+	t->haspointers = 0;
 	dowidth(t);
 
 	if(ctxt != 0) {