Xwayland creates and destroys the CRTC along with the Wayland outputs,
so there is possibly a case where the number of CRTC drops to 0.
However, `xwl_present_get_crtc()` always return `crtcs[0]` which is
invalid when `numCrtcs` is 0.
That leads to crash if a client queries the Present capabilities when
there is no CRTC, the backtrace looks like:
#0 raise() from libc.so
#1 abort() from libc.so
#2 OsAbort() at utils.c:1350
#3 AbortServer() at log.c:879
#4 FatalError() at log.c:1017
#5 OsSigHandler() at osinit.c:156
#6 OsSigHandler() at osinit.c:110
#7 <signal handler called>
#8 main_arena() from libc.so
#9 proc_present_query_capabilities() at present_request.c:236
#10 Dispatch() at dispatch.c:478
#11 dix_main() at main.c:276
To avoid returning an invalid pointer (`crtcs[0]`) in that case, simply
check for `numCrtcs` being 0 and return `NULL` in that case.
Thanks to Michel Dänzer <michel.daenzer@amd.com> for pointing this as a
possible cause of the crash.
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Bugzilla: https://bugzilla.redhat.com/1609181
e8295c5020