Calling userHasLeft before sending the leave presence may result in
invalid state as the MUC presence lister may modify the MUCs local
state, e.g., occupantsMap, conurrently with the leave operation.
If we reset it after the leave presence was send and acknowledged,
then this can not happen as the server will not longer send any MUC
related presences to us.
Also fixes SMACK-914. In theory 52a49769f9 ("[muc] Check for
self-presence first in presence listener") alone would fix SMACK-914,
but this also fixes it indepentendly of 52a49769f9. Both commits are
sensible, so both are applied.
105c74b22b