Fix crash on adding images to right-to-left mail messages. Bug 155096, r=mjudge, sr=kin

content/base/src/nsSelection.cpp

@@ -2350,6 +2350,9 @@ nsSelection::GetPrevNextBidiLevels(nsIPresContext *aPresContext,
   if (NS_FAILED(result))
     return result;
 
+  if (thisLine < 0) 
+    return NS_ERROR_FAILURE;
+
   nsIFrame *firstFrame;
   nsIFrame *lastFrame;
   nsRect    nonUsedRect;

layout/generic/nsSelection.cpp

@@ -2350,6 +2350,9 @@ nsSelection::GetPrevNextBidiLevels(nsIPresContext *aPresContext,
   if (NS_FAILED(result))
     return result;
 
+  if (thisLine < 0) 
+    return NS_ERROR_FAILURE;
+
   nsIFrame *firstFrame;
   nsIFrame *lastFrame;
   nsRect    nonUsedRect;