Merge pull request #59 from mattn/fix_issue56

fix #56

src/variable.c

@@ -347,11 +347,13 @@ mrb_f_global_variables(mrb_state *mrb, mrb_value self)
   struct kh_iv *h = mrb->globals;
   mrb_value ary = mrb_ary_new(mrb);
 
-  for (i=0;i< kh_end(h);i++) {
+  if (h) {
+    for (i=0;i < kh_end(h);i++) {
       if (kh_exist(h, i)) {
         mrb_ary_push(mrb, ary, mrb_symbol_value(kh_key(h,i)));
       }
     }
+  }
   buf[0] = '$';
   buf[2] = 0;
   for (i = 1; i <= 9; ++i) {