Bug 830214 - Avoid crashing when printing a document with many mozPrintCallback canvases. r=roc

--HG--
extra : rebase_source : 98ae3180cb36f040241ae0b6850485923afe4570

layout/generic/nsSimplePageSequence.cpp

@@ -633,6 +633,10 @@ nsSimplePageSequenceFrame::PrePrintNextPage(nsITimerCallback* aCallback, bool* a
              size
            );
 
+        if (!printSurface) {
+          continue;
+        }
+
         nsICanvasRenderingContextInternal* ctx = canvas->GetContextAtIndex(0);
 
         if (!ctx) {