h264_metadata: Fix double-free

Whether the udu string should be freed depends on whether the SEI it
gets added to was created internally by cbs or externally by the bsf.
The current code frees it twice in the former case.

libavcodec/h264_metadata_bsf.c

@@ -293,7 +293,7 @@ static int h264_metadata_filter(AVBSFContext *bsf, AVPacket *out)
         H264RawSEI *sei;
         H264RawSEIPayload *payload;
         H264RawSEIUserDataUnregistered *udu;
-        int sei_pos;
+        int sei_pos, sei_new;
 
         for (i = 0; i < au->nb_units; i++) {
             if (au->units[i].type == H264_NAL_SEI ||
@@ -305,8 +305,10 @@ static int h264_metadata_filter(AVBSFContext *bsf, AVPacket *out)
 
         if (sei_pos < au->nb_units &&
             au->units[sei_pos].type == H264_NAL_SEI) {
+            sei_new = 0;
             sei = au->units[sei_pos].content;
         } else {
+            sei_new = 1;
             sei = &ctx->sei_nal;
             memset(sei, 0, sizeof(*sei));
 
@@ -354,6 +356,12 @@ static int h264_metadata_filter(AVBSFContext *bsf, AVPacket *out)
 
             payload->payload_size = 16 + udu->data_length;
 
+            if (!sei_new) {
+                // This will be freed by the existing internal
+                // reference in fragment_uninit().
+                sei_udu_string = NULL;
+            }
+
         } else {
         invalid_user_data:
             av_log(bsf, AV_LOG_ERROR, "Invalid user data: "