gfs2: Silence possible null pointer dereference warning

In gfs2_rbm_find, rs is always NULL when minext is NULL, so
gfs2_reservation_check_and_update will never be called on a NULL minext.
This isn't innediately obvious though, so also check for a NULL minext
for better code readability.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/rgrp.c

@@ -1783,7 +1783,7 @@ static int gfs2_rbm_find(struct gfs2_rbm *rbm, u8 state, u32 *minext,
 			goto next_bitmap;
 		}
 		rbm->offset = offset;
-		if (!rs)
+		if (!rs || !minext)
 			return 0;
 
 		ret = gfs2_reservation_check_and_update(rbm, rs, *minext,