backend/x11: prevent segfault on empty DRI3 response

When trying to launch a wlroots-based compositor using X11 forwarding via SSH, it segfaults because dri3_reply is null

It's been also an issue in Mesa, successfully patched in mesa/mesa!15469 (merged)

Partial gdb log:

(gdb) run
Starting program: /usr/bin/sway 
[Detaching after vfork from child process 9823]

Program received signal SIGSEGV, Segmentation fault.
wlr_x11_backend_create (display=display@entry=0x7ffff7bcde80, x11_display=<optimized out>) at ../backend/x11/backend.c:458
458				wlr_log(WLR_INFO, "X11 does not support required DRI3 version "
(gdb) bt full
#0  wlr_x11_backend_create (display=display@entry=0x7ffff7bcde80, x11_display=<optimized out>) at ../backend/x11/backend.c:458
        dri3_cookie = <optimized out>
        dri3_reply = 0x0
        x11 = 0x7ffff79f0920