compositor-x11: free or discard XCB generic_error's
compositor-x11: free or discard XCB generic_error's

Where we don't look at the error details, pass NULL to the 'error'
argument and test using the reply return value instead.
Where we do need it, remember to free it.

Signed-off-by: Ran Benita <03686f031484546e32da38cdd36b1b8bb5e2be3c@gmail.com>

sir-murray/weston,eyolfson/weston,giucam/weston,mchalupa/weston,Gnurou/weston,xorgy/weston,Fantu/compositor-spice,udoprog/weston,giucam/weston,Tarnyko/weston-xdg_surface_present,kwm81/weston,xorgy/weston,udoprog/weston,Gnurou/weston,krezovic/weston,Tarnyko/weston-xdg_surface_present,krezovic/weston,Fantu/compositor-spice,eyolfson/weston,mchalupa/weston,jonnylamb/weston,Fantu/compositor-spice,kwm81/weston,jonnylamb/weston,sir-murray/weston