interp: fix implicit type for bin composite lit case

When the type is implicit, the first element in the list of children is
not the type of the composite literal, but it is one of the actual
children, so it should not be discarded.

Fixes #862

interp/run.go

@@ -2010,12 +2010,15 @@ func compositeBinMap(n *node) {
 	}
 }
 
-// compositeBinStruct creates and populates a struct object from a binary type.
-func compositeBinStruct(n *node) {
+// doCompositeBinStruct creates and populates a struct object from a binary type.
+func doCompositeBinStruct(n *node, hasType bool) {
 	next := getExec(n.tnext)
 	value := valueGenerator(n, n.findex)
 	typ := n.typ.rtype
-	child := n.child[1:]
+	child := n.child
+	if hasType {
+		child = n.child[1:]
+	}
 	values := make([]func(*frame) reflect.Value, len(child))
 	fieldIndex := make([][]int, len(child))
 	for i, c := range child {
@@ -2051,6 +2054,9 @@ func compositeBinStruct(n *node) {
 	}
 }
 
+func compositeBinStruct(n *node)       { doCompositeBinStruct(n, true) }
+func compositeBinStructNotype(n *node) { doCompositeBinStruct(n, false) }
+
 func destType(n *node) *itype {
 	switch n.anc.kind {
 	case assignStmt, defineStmt: