revert usage of GGML_ASSERT

Author: ngxson

examples/server/server.cpp

@@ -1837,8 +1837,10 @@ struct server_context {
             if (slot.ga_n == 1) {
                 if (slot.is_processing() && (int) system_tokens.size() + slot.n_past >= slot.n_ctx - 1) {
                     if (!params.ctx_shift) {
+                        // this check is redundant (for good)
                         // we should never get here, because generation should already stopped in process_token()
-                        GGML_ASSERT(false && "context shifting is disabled");
+                        slot.release();
+                        send_error(slot, "context shift is disabled", ERROR_TYPE_SERVER);
                         continue;
                     }