Followon from a checkin I did yesterday to fix bugs 29521 and 29827 (a=jar); this part fixes an assertion that happened with empty form fields. r=kin

xpcom/io/nsLinebreakConverter.cpp

@@ -453,6 +453,9 @@ nsresult nsLinebreakConverter::ConvertStringLineBreaks(nsString& ioString,
 
   NS_ASSERTION(aDestBreaks != eLinebreakAny, "Invalid parameter");
 
+  // nothing to do
+  if (ioString.Length() == 0) return NS_OK;
+  
   // we can't go messing with data we don't own
   if (!ioString.mOwnsBuffer) return NS_ERROR_UNEXPECTED;