Add `argv` and `argc` check in `OP_ENTER`; fix #4102

`argv` may be retrieved from an array whose `ptr` is `NULL` when it`s
empty.

src/vm.c

@@ -1791,21 +1791,19 @@ RETRY_TRY_BLOCK:
           kargs = 0;
         }
         else {
-          if (!mrb_hash_p(argv[argc - 1])) {
-            if (r) {
-              kdict = mrb_hash_new(mrb);
-              kargs = 0;
-            }
-            else {
-              mrb_value str = mrb_str_new_lit(mrb, "Excepcted `Hash` as last argument for keyword arguments");
-              mrb_exc_set(mrb, mrb_exc_new_str(mrb, E_ARGUMENT_ERROR, str));
-              goto L_RAISE;
-            }
+          if (argv && argc > 0 && mrb_hash_p(argv[argc-1])) {
+            kdict = argv[argc-1];
+            mrb_hash_check_kdict(mrb, kdict);
+          }
+          else if (r) {
+            kdict = mrb_hash_new(mrb);
+            kargs = 0;
           }
           else {
-            kdict = argv[argc-1];
+            mrb_value str = mrb_str_new_lit(mrb, "Excepcted `Hash` as last argument for keyword arguments");
+            mrb_exc_set(mrb, mrb_exc_new_str(mrb, E_ARGUMENT_ERROR, str));
+            goto L_RAISE;
           }
-          mrb_hash_check_kdict(mrb, kdict);
           if (MRB_ASPEC_KEY(a) > 0) {
             kdict = mrb_hash_dup(mrb, kdict);
           }