fftools/ffmpeg: use a mutex for enc_stats_write()

It may be called concurrently from different threads to write into the
same file.

fftools/ffmpeg_mux_init.c

@@ -402,6 +402,11 @@ fail:
             return ret;
     }
 
+    ret = pthread_mutex_init(&es->lock, NULL);
+    if (ret)
+        return AVERROR(ret);
+    es->lock_initialized = 1;
+
     ret = enc_stats_get_file(&es->io, path);
     if (ret < 0)
         return ret;