fix: add null check for clip buffer in reset clip command

Co-authored-by: aider (deepseek/deepseek-coder) <aider@aider.chat>

engine.c

@@ -364,6 +364,7 @@ void engine_process_commands(Engine *engine) {
             case CMD_RESET_CLIP: {
                 if (cmd.index < 0 || cmd.index >= MAX_CLIPS) break;
                 Clip *clip = &engine->clips[cmd.index];
+                if (!clip->buffer) break;  // ADD THIS - prevent segfault on freed buffer
                 
                 // Record undo action
                 UndoAction action;