Bug 1400374 - Fix compiler warning in FixGrayBits. r=smaug

The compiler complains because a postfix increment is done inside of an assert.

MozReview-Commit-ID: LSRsWMn9Tlj

--HG--
extra : rebase_source : 18539a430c3d84675c602699be9a5a62dd477fec

xpcom/base/nsCycleCollector.cpp

@@ -3595,7 +3595,8 @@ nsCycleCollector::FixGrayBits(bool aForceGC, TimeLog& aTimeLog)
     // It's possible that FixWeakMappingGrayBits will hit OOM when unmarking
     // gray and we will have to go round again. The second time there should not
     // be any weak mappings to fix up so the loop body should run at most twice.
-    MOZ_RELEASE_ASSERT(count++ < 2);
+    MOZ_RELEASE_ASSERT(count < 2);
+    count++;
   } while (!mCCJSRuntime->AreGCGrayBitsValid());
 
   aTimeLog.Checkpoint("FixGrayBits");