libceph: potential NULL dereference in ceph_osdc_handle_map()

There are two places where we read "nr_maps" if both of them are set to
zero then we would hit a NULL dereference here.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Sage Weil <sage@inktank.com>
diff --git a/net/ceph/osd_client.c b/net/ceph/osd_client.c
index 3ee4a1d..6a59fb8 100644
--- a/net/ceph/osd_client.c
+++ b/net/ceph/osd_client.c
@@ -1793,6 +1793,8 @@
 		nr_maps--;
 	}
 
+	if (!osdc->osdmap)
+		goto bad;
 done:
 	downgrade_write(&osdc->map_sem);
 	ceph_monc_got_osdmap(&osdc->client->monc, osdc->osdmap->epoch);