Bug 1278201 - prevent writing |mRefCnt| after it's freed. r=Ms2ger

MozReview-Commit-ID: G7fZzrGWHmW

--HG--
extra : rebase_source : 6f311948a8d2222358aec860f2c6d6223862e814

mfbt/tests/TestNotNull.cpp

@@ -67,10 +67,11 @@ public:
 
   uint32_t Release() {
     CHECK(mRefCnt > 0);
-    if (mRefCnt == 1) {
-      delete this;
-    }
     mRefCnt--;
+    if (mRefCnt == 0) {
+      delete this;
+      return 0;
+    }
     return mRefCnt;
   }
 };