Free the original pointer if `realloc` failed.

The POSIX `realloc` keep the original pointer untouched, so it can
easily leads to memory leakage. `mrb_realloc()` should handle those
bookkeeping, while `mrb_realloc_simple()` keeps the original `realloc`
behavior.

src/gc.c

@@ -225,14 +225,9 @@ mrb_realloc(mrb_state *mrb, void *p, size_t len)
   p2 = mrb_realloc_simple(mrb, p, len);
   if (len == 0) return p2;
   if (p2 == NULL) {
-    if (mrb->gc.out_of_memory) {
-      mrb_raise_nomemory(mrb);
-      /* mrb_panic(mrb); */
-    }
-    else {
-      mrb->gc.out_of_memory = TRUE;
-      mrb_raise_nomemory(mrb);
-    }
+    mrb_free(mrb, p);
+    mrb->gc.out_of_memory = TRUE;
+    mrb_raise_nomemory(mrb);
   }
   else {
     mrb->gc.out_of_memory = FALSE;