libx264: Make codec use the init-cleanup flag and mark it as init-thread-safe

This takes care of memory leaks on init error.

libavcodec/libx264.c

@@ -272,10 +272,12 @@ static av_cold int X264_close(AVCodecContext *avctx)
     X264Context *x4 = avctx->priv_data;
 
     av_freep(&avctx->extradata);
-    av_free(x4->sei);
+    av_freep(&x4->sei);
 
-    if (x4->enc)
+    if (x4->enc) {
         x264_encoder_close(x4->enc);
+        x4->enc = NULL;
+    }
 
     av_frame_free(&avctx->coded_frame);
 
@@ -697,4 +699,6 @@ AVCodec ff_libx264_encoder = {
     .priv_class       = &class,
     .defaults         = x264_defaults,
     .init_static_data = X264_init_static,
+    .caps_internal    = FF_CODEC_CAP_INIT_THREADSAFE |
+                        FF_CODEC_CAP_INIT_CLEANUP,
 };