wsi/x11: Fix possible null pointer deference

Fix defect CID-1476002 reported by Coverity Scan.

wsi_x11_get_connection() could return NULL in case of memory allocation
failure. Added 'if' to catch this case, as it is done in all other
usages of this function.

Fixes: mesa/mesa@dde2e324

Signed-off-by: Yevhenii Kharchenko <yevhenii.kharchenko@globallogic.com>