Bug 739604 - Check if output file is valid before printing. r=mattwoodrow

Check that gfxUtils::sDumpPaintFile isn't NULL before calling print functions
in Layers.cpp:WriteSnapshotToDumpFile_internal().

gfx/layers/Layers.cpp

@@ -572,8 +572,10 @@ void WriteSnapshotToDumpFile_internal(T* aObj, gfxASurface* aSurf)
   nsCString string(aObj->Name());
   string.Append("-");
   string.AppendInt((PRUint64)aObj);
+  if (gfxUtils::sDumpPaintFile)
     fprintf(gfxUtils::sDumpPaintFile, "array[\"%s\"]=\"", string.BeginReading());
   aSurf->DumpAsDataURL(gfxUtils::sDumpPaintFile);
+  if (gfxUtils::sDumpPaintFile)
     fprintf(gfxUtils::sDumpPaintFile, "\";");
 }