Fixed assert when delete[]-ing an array of size zero.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@7755 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/exp-drd/drd_malloc_wrappers.c b/exp-drd/drd_malloc_wrappers.c
index b2d06cd..4fb6310 100644
--- a/exp-drd/drd_malloc_wrappers.c
+++ b/exp-drd/drd_malloc_wrappers.c
@@ -143,7 +143,8 @@
   }
   else
   {
-    s_stop_using_mem_callback(mc->data, mc->size);
+    if (mc->size > 0)
+      s_stop_using_mem_callback(mc->data, mc->size);
     VG_(free)(mc);
   }
 }