Xserver/os/xdmauth.c: Avoid compiler warnings due to -Wswitch being enabled. Use if statement rather than switch.

 Backported from X.org. Found in commit...

 commit 7deaaa797cf8e7ca71e9b34fa6f413d1ed2b3dab
 Author: Adam Jackson <ajax@nwnk.net>
 Date:   Tue Mar 28 01:21:00 2006 +0000