making sure that we do not empty our segmented buffer if we did not create it

bug fix 19290.  submitted by bienvenu, r=dougt.

xpcom/io/nsIFileStream.cpp

@@ -474,6 +474,7 @@ NS_IMETHODIMP FileImpl::Flush()
         }
     }
 
+    if (mGotBuffers)
         mOutBuffer.Empty();
     mWriteCursor = nsnull;
     mWriteLimit  = nsnull;