fix: don't panic in map range if assigned value is _

Ensure that the code generated for `for s, _ = range ...`
is the same as `for s = range ...`.

Fixes 1622.

_test/map31.go

@@ -0,0 +1,13 @@
+package main
+
+func main() {
+	myMap := map[string]int{"a":2}
+
+	for s, _ := range myMap {
+		_ = s
+	}
+	println("ok")
+}
+
+// Output:
+// ok

interp/cfg.go

@@ -147,6 +147,9 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 					var k, v, o *node
 					if len(n.anc.child) == 4 {
 						k, v, o = n.anc.child[0], n.anc.child[1], n.anc.child[2]
+						if v.ident == "_" {
+							v = nil // Do not assign to _ value.
+						}
 					} else {
 						k, o = n.anc.child[0], n.anc.child[1]
 					}

interp/run.go

@@ -2901,11 +2901,10 @@ func rangeMap(n *node) {
 	index2 := index0 - 1        // iterator for range, always just behind index0
 	fnext := getExec(n.fnext)
 	tnext := getExec(n.tnext)
+	value := genValue(n.child[len(n.child)-2]) // map value
 
-	var value func(*frame) reflect.Value
-	if len(n.child) == 4 {
-		index1 := n.child[1].findex  // map value location in frame
-		value = genValue(n.child[2]) // map
+	if len(n.child) == 4 && n.child[1].ident != "_" {
+		index1 := n.child[1].findex // map value location in frame
 		n.exec = func(f *frame) bltn {
 			iter := f.data[index2].Interface().(*reflect.MapIter)
 			if !iter.Next() {
@@ -2916,7 +2915,6 @@ func rangeMap(n *node) {
 			return tnext
 		}
 	} else {
-		value = genValue(n.child[1]) // map
 		n.exec = func(f *frame) bltn {
 			iter := f.data[index2].Interface().(*reflect.MapIter)
 			if !iter.Next() {