ephyr: Avoid a segfault with 'DISPLAY= Xephy -glamor'

ephyr_glamor_connect() returns NULL if we failed, but applying
xcb_connection_has_error() to NULL is not permitted.

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Daniel Martin <consume.noise@gmail.com>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>
(cherry picked from commit f42520c5f1)

hw/kdrive/ephyr/hostx.c

@@ -443,7 +443,7 @@ hostx_init(void)
     else
 #endif
         HostX.conn = xcb_connect(NULL, &HostX.screen);
-    if (xcb_connection_has_error(HostX.conn)) {
+    if (!HostX.conn || xcb_connection_has_error(HostX.conn)) {
         fprintf(stderr, "\nXephyr cannot open host display. Is DISPLAY set?\n");
         exit(1);
     }