cmd/gc: fix divide by zero error in compiler

Fixes #6399.

R=ken2
CC=golang-dev
https://golang.org/cl/13253055

src/cmd/gc/walk.c

@@ -1295,7 +1295,7 @@ walkexpr(Node **np, NodeList **init)
 		t = n->type;
 		if(n->esc == EscNone
 			&& smallintconst(l) && smallintconst(r)
-			&& mpgetfix(r->val.u.xval) < (1ULL<<16) / t->type->width) {
+			&& (t->type->width == 0 || mpgetfix(r->val.u.xval) < (1ULL<<16) / t->type->width)) {
 			// var arr [r]T
 			// n = arr[:l]
 			t = aindex(r, t->type); // [r]T

test/fixedbugs/issue6399.go

+// compile
+
+package main
+
+type Foo interface {
+	Print()
+}
+
+type Bar struct{}
+
+func (b Bar) Print() {}
+
+func main() {
+	b := make([]Bar, 20)
+	f := make([]Foo, 20)
+	for i := range f {
+		f[i] = b[i]
+	}
+	T(f)
+	_ = make([]struct{}, 1)
+}
+
+func T(f []Foo) {
+	for i := range f {
+		f[i].Print()
+	}
+}