Don't fall off the edge without returning a value. b=514689 r=fantasai

layout/generic/nsBlockFrame.cpp

@@ -5682,6 +5682,7 @@ nsBlockFrame::FindTrailingClear()
       return endLine->GetBreakTypeAfter();
     }
   }
+  return NS_STYLE_CLEAR_NONE;
 }
 
 nsresult