fix no content-type in source mbox

src/cpp/MainAttachment.cpp

@@ -491,6 +491,8 @@ void MainAttachment::rewriteHeaders (ifstream &mbox, ofstream &outbox, const Hea
     boundary = "--"+boundary+"--";
     boundaryMiddleSize = boundary.length () - 2;
   }
+  if (startPos >= mime.length ())
+    startPos = mime.length ()-1;
   mime.insert (startPos, contentType);
   outbox << mime << flush;
 }