Bug 452797 - Clean up unused variables in layout/; (debug) bustage fix; noticed by Jesse on irc.

layout/generic/nsContainerFrame.cpp

@@ -1099,7 +1099,11 @@ void
 nsContainerFrame::DeleteNextInFlowChild(nsPresContext* aPresContext,
                                         nsIFrame*      aNextInFlow)
 {
-  NS_PRECONDITION(aNextInFlow->GetPrevInFlow(), "bad prev-in-flow");
+#ifdef DEBUG
+  nsIFrame* prevInFlow = aNextInFlow->GetPrevInFlow();
+#endif
+
+  NS_PRECONDITION(prevInFlow, "bad prev-in-flow");
 
   // If the next-in-flow has a next-in-flow then delete it, too (and
   // delete it first).