Fix segfault on empty file recorded

Write the file trailer only if the file header have been written, to
avoid a segfault in libav.

Fixes <https://github.com/Genymobile/scrcpy/issues/918>.

app/src/recorder.c

@@ -174,11 +174,16 @@ recorder_open(struct recorder *recorder, const AVCodec *input_codec) {
 
 void
 recorder_close(struct recorder *recorder) {
+    if (recorder->header_written) {
         int ret = av_write_trailer(recorder->ctx);
         if (ret < 0) {
             LOGE("Failed to write trailer to %s", recorder->filename);
             recorder->failed = true;
         }
+    } else {
+        // the recorded file is empty
+        recorder->failed = true;
+    }
     avio_close(recorder->ctx->pb);
     avformat_free_context(recorder->ctx);