avcodec: move end zeroing code from av_packet_split_side_data() to avcodec_decode_subtitle2()

This code changes the input packet, which is read only and can in
rare circumstances lead to decoder errors. (i run into one of these in
the audio decoder, which corrupted the packet during av_find_stream_info()
so that actual decoding that single packet failed later)
Until a better fix is implemented, this commit limits the problem.
A better fix might be to make the subtitle decoders not depend on
data[size] = 0 or to copy their input when this is not the case.
