librlist: fix use-after-free after rlist_remove_ranks()

Problem: rlist_remove_rank() removes the requested rnode from the rlist
nodes list, but doesn't delete the item from the rank hash.  This can
lead to a use-after-free if the removed rank is looked up after this.

Call rank_hash_delete() after the rank is deleted from the nodes list
to ensure references to the removed rnode.

Author: grondo

src/common/librlist/rlist.c

@@ -398,6 +398,7 @@ static int rlist_remove_rank (struct rlist *rl, int rank)
         errno = ENOENT;
         return -1;
     }
+    rank_hash_delete (rl, rank);
     zlistx_delete (rl->nodes, handle);
     return 0;
 }