avcodec/decode: Fix leaks upon subtitle decoding errors

Up until now, various subtitle decoders have not cleaned up
the AVSubtitle on error; this task must not be left to the user
because the documentation explicitly states that the AVSubtitle
"must be freed with avsubtitle_free if *got_sub_ptr is set"
(which it isn't on error).
Leaks happen upon failure in ff_ass_add_rect() or in
ass_decode_frame(); freeing generically also allows to remove
now redundant freeing code in pgssubdec and dvbsubdec.
While just at it, also reset got_sub_ptr generically on error.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/decode.c

@@ -834,8 +834,14 @@ int avcodec_decode_subtitle2(AVCodecContext *avctx, AVSubtitle *sub,
             sub->pts = av_rescale_q(avpkt->pts,
                                     avctx->pkt_timebase, AV_TIME_BASE_Q);
         ret = avctx->codec->decode(avctx, sub, got_sub_ptr, pkt);
-        av_assert1((ret >= 0) >= !!*got_sub_ptr &&
-                   !!*got_sub_ptr >= !!sub->num_rects);
+        if (pkt == avci->buffer_pkt) // did we recode?
+            av_packet_unref(avci->buffer_pkt);
+        if (ret < 0) {
+            *got_sub_ptr = 0;
+            avsubtitle_free(sub);
+            return ret;
+        }
+        av_assert1(!sub->num_rects || *got_sub_ptr);
 
         if (sub->num_rects && !sub->end_display_time && avpkt->duration &&
             avctx->pkt_timebase.num) {
@@ -863,9 +869,6 @@ int avcodec_decode_subtitle2(AVCodecContext *avctx, AVSubtitle *sub,
 
         if (*got_sub_ptr)
             avctx->frame_number++;
-
-        if (pkt == avci->buffer_pkt) // did we recode?
-            av_packet_unref(avci->buffer_pkt);
     }
 
     return ret;

libavcodec/dvbsubdec.c

@@ -1715,8 +1715,6 @@ static int dvbsub_decode(AVCodecContext *avctx,
 
 end:
     if (ret < 0) {
-        *got_sub_ptr = 0;
-        avsubtitle_free(sub);
         return ret;
     } else {
         if (ctx->compute_edt == 1)

libavcodec/pgssubdec.c

@@ -667,11 +667,8 @@ static int decode(AVCodecContext *avctx, void *data, int *got_sub_ptr,
             break;
         }
         if (ret < 0 && (ret == AVERROR(ENOMEM) ||
-                        avctx->err_recognition & AV_EF_EXPLODE)) {
-            avsubtitle_free(data);
-            *got_sub_ptr = 0;
+                        avctx->err_recognition & AV_EF_EXPLODE))
             return ret;
-        }
 
         buf += segment_length;
     }