avcodec/gif: Fix leaks upon allocation error

If one of several allocations the gif encoder performs in its init
function fails, the successful allocations leak. Fix this by adding the
FF_CODEC_CAP_INIT_CLEANUP flag.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 8da8774d8e)

libavcodec/gif.c

@@ -497,4 +497,5 @@ AVCodec ff_gif_encoder = {
         AV_PIX_FMT_GRAY8, AV_PIX_FMT_PAL8, AV_PIX_FMT_NONE
     },
     .priv_class     = &gif_class,
+    .caps_internal  = FF_CODEC_CAP_INIT_CLEANUP,
 };