XCB: silence warnings about possibly undefined shifts

The code shifts "1" by a uint32 number. If that number is >= 32,
we're triggering undefined behavior, and Coverity rightfully complains
about that. Add some asserts to silence those warnings.

Change-Id: Ib91085a279b0a2b7ad37afad05ec1d764c0496b1
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>