egl/x11: bail out if we cannot fetch the xcb connection

egl/x11: bail out if we cannot fetch the xcb connection

The documentation of xcb_connection_has_error() does not mention
what will happen, if NULL is fed to the function.

Upon closer look (props to Matt), it seems that we'll crash as the
implementation dereferences conn.

This will also allow us to remove the dri2_dpy->conn checking with the
next commit.

v2: Reword commit message as per Matt's findings.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>