MEDIUM: mux-pt: Don't always set a final error on SE on the sending path

SE_FL_ERROR must be set on the SE descriptor only if EOS was already
reported. So call se_fl_set_error() function to properly the
ERR_PENDING/ERROR flags. It is not really a bug because the mux-pt is really
simple. But it is better to do it now the right way.
1 file changed