interp: fix litteral map containing binary functions

The case of assigning a binary function to a funcT object was
solved elsewhere. Factor the case in genDestValue to apply it
at multiple places.

Fixes #1100.

interp/type.go

@@ -1723,7 +1723,7 @@ func isInterfaceSrc(t *itype) bool {
 }
 
 func isInterfaceBin(t *itype) bool {
-	return t.cat == valueT && t.rtype.Kind() == reflect.Interface
+	return t.cat == valueT && t.rtype.Kind() == reflect.Interface || t.cat == errorT
 }
 
 func isInterface(t *itype) bool {