avcodec/libx264: Fix leak in case of allocation failure

Fixes Coverity issue #1518906.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Reviewed-by: Jan Ekström <jeebjp@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/libx264.c

@@ -503,6 +503,7 @@ FF_ENABLE_DEPRECATION_WARNINGS
         if (sei_data) {
             pic->extra_sei.payloads = av_mallocz(sizeof(pic->extra_sei.payloads[0]));
             if (pic->extra_sei.payloads == NULL) {
+                av_free(sei_data);
                 ret = AVERROR(ENOMEM);
                 goto fail;
             }