bcachefs: Fix a null ptr deref in bch2_get_alloc_in_memory_pos()

Reported-by: smatch
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/backpointers.c

@@ -703,7 +703,7 @@ static int bch2_get_alloc_in_memory_pos(struct btree_trans *trans,
 
 		--btree_nodes;
 		if (!btree_nodes) {
-			*end = alloc_k.k->p;
+			*end = alloc_k.k ? alloc_k.k->p : SPOS_MAX;
 			break;
 		}