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>