Fix missing prev update in hnsw_cursor_free causing unlink failure (#14050)

This PR fixes a bug in the `hnsw_cursor_free` function where the prev
pointer was never updated during cursor list traversal. As a result, if
the cursor being freed was not the head of the list, it would not be
correctly unlinked, potentially causing memory leaks or corruption of
the cursor list.

Note that since `hnsw_cursor_free()` is never used for now, this PR does
not actually fix any bug.

modules/vector-sets/hnsw.c

@@ -2489,6 +2489,7 @@ void hnsw_cursor_free(hnswCursor *cursor) {
             hfree(cursor);
             break;
         }
+        prev = x;
         x = x->next;
     }
     pthread_rwlock_unlock(&cursor->index->global_lock);