lavc/cbs_h2645_syntax_template: Fix memleak

payload_count is used to track the number of SEI payloads. It is also
used to free the SEIs in cbs_h264_free_sei()/cbs_h265_free_sei().

Currently, payload_count is set after for loop is completed. Hence if
there is an error and the function exits, the payload remains zero
causing a memleak.

This commit keeps track of payload_count inside the for loop to fix the
issue. Note that that the contents of current are initialized with
av_mallocz() so there is no need to zero initialize payload_count.

Found-by: libFuzzer
Reviewed-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>

libavcodec/cbs_h264_syntax_template.c

@@ -954,6 +954,7 @@ static int FUNC(sei)(CodedBitstreamContext *ctx, RWContext *rw,
         current->payload[k].payload_type = payload_type;
         current->payload[k].payload_size = payload_size;
 
+        current->payload_count++;
         CHECK(FUNC(sei_payload)(ctx, rw, &current->payload[k]));
 
         if (!cbs_h2645_read_more_rbsp_data(rw))
@@ -964,7 +965,6 @@ static int FUNC(sei)(CodedBitstreamContext *ctx, RWContext *rw,
                "SEI message: found %d.\n", k);
         return AVERROR_INVALIDDATA;
     }
-    current->payload_count = k + 1;
 #else
     for (k = 0; k < current->payload_count; k++) {
         PutBitContext start_state;

libavcodec/cbs_h265_syntax_template.c

@@ -2184,6 +2184,7 @@ static int FUNC(sei)(CodedBitstreamContext *ctx, RWContext *rw,
         current->payload[k].payload_type = payload_type;
         current->payload[k].payload_size = payload_size;
 
+        current->payload_count++;
         CHECK(FUNC(sei_payload)(ctx, rw, &current->payload[k], prefix));
 
         if (!cbs_h2645_read_more_rbsp_data(rw))
@@ -2194,7 +2195,6 @@ static int FUNC(sei)(CodedBitstreamContext *ctx, RWContext *rw,
                "SEI message: found %d.\n", k);
         return AVERROR_INVALIDDATA;
     }
-    current->payload_count = k + 1;
 #else
     for (k = 0; k < current->payload_count; k++) {
         PutBitContext start_state;