Better check for lib has only functions (fix #138)

execute.c

@@ -315,6 +315,8 @@ jv jq_next(jq_state *jq) {
     switch (opcode) {
     default: assert(0 && "invalid instruction");
 
+    case TOP: break;
+
     case LOADK: {
       jv v = jv_array_get(jv_copy(frame_current(jq)->bc->constants), *pc++);
       assert(jv_is_valid(v));