Bug 1361612: Initialize nsPlaceholderFrame::mOutOfFlowFrame. r=mats

MozReview-Commit-ID: CZoX9t8noAv
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

layout/generic/nsPlaceholderFrame.h

@@ -166,7 +166,7 @@ public:
   }
 
 protected:
-  nsIFrame* mOutOfFlowFrame;
+  nsIFrame* mOutOfFlowFrame { nullptr };
 };
 
 #endif /* nsPlaceholderFrame_h___ */