llama : fix edge case finding batch seq_id of split recurrent cell

This otherwise was a problem when running the HellaSwag benchmark
with small batch sizes, making it crash.

Author: compilade

llama.cpp

@@ -3879,11 +3879,17 @@ static bool llama_cache_find_slot(
                         if (cell.tail_rc == 0) {
                             cache.rs.clear_cell(cell);
                         } else {
-                            // TODO: does this always work correctly
-                            // even if there are more than one seq_node in this cell?
+                            // Find the seq_id of the first tail of this cell
+                            llama_seq_id seq_id = -1;
+                            for (llama_rs_seq_node & seq_node : cell.seq_nodes) {
+                                if (seq_node.is_tail()) {
+                                    seq_id = seq_node.seq_id;
+                                    break;
+                                }
+                            }
+                            GGML_ASSERT(seq_id != -1);
 
                             // Which seq_id of the batch is it?
-                            llama_seq_id seq_id = cell.seq_nodes[0].seq_id;
                             int32_t nth_seq_id = -1;
                             for (int32_t s = 0; (uint32_t) s < n_seqs; ++s) {
                                 if (seq_id == batch.seq_id[s][0]) {