mupdate-client: Fix segfault due to use after free condition in mupdate-client.
The libsasl callback data structure is freed after connection
initialisation, but used during the whole connection. This leads to
arbitrary segfaults, when malloc() eventually reclaims the memory
area.
This commit defers the memory release until disconnection.
Signed-off-by: Mario Haustein <mario.haustein@hrz.tu-chemnitz.de>