delete mrb_free()-related non-NULL checks

No need to optimize since a program only exits once and errors are rare.
Also the mruby source code doesn't have these kind of checks elsewhere.

The ones in {Time,Random}#initialize are kept because there it actually
matters
since initialization always happens and re-initialization is unlikely.

src/dump.c

@@ -978,12 +978,8 @@ error_exit:
     mrb_free(mrb, *bin);
     *bin = NULL;
   }
-  if (lv_syms) {
-    mrb_free(mrb, lv_syms);
-  }
-  if (filenames) {
-    mrb_free(mrb, filenames);
-  }
+  mrb_free(mrb, lv_syms);
+  mrb_free(mrb, filenames);
   return result;
 }