MySQL: add decode connattrs when use COM_CHANGE_USER

Change-Id: I5c24f0bf3d756abed0db17bf2ae8ab0dbf695c43
Ping-Bug:10351
Reviewed-on: https://code.wireshark.org/review/4000
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>