subtitles: Don't manage the buffer assigned to a packet.

Remove code that attempted to have FFmpeg reuse the same buffer each
time it read a subtitle.  The FFmpeg code changes out the packet
buffer each time av_read_frame is called.

Refs #767.

mythtv/libs/libmythtv/captions/textsubtitleparser.cpp

@@ -175,7 +175,6 @@ TextSubtitleParser::TextSubtitleParser(SubtitleReader *parent, QString fileName,
     : m_parent(parent), m_target(target), m_fileName(std::move(fileName))
 {
     m_pkt = av_packet_alloc();
-    av_new_packet(m_pkt, 4096);
 }
 
 TextSubtitleParser::~TextSubtitleParser()
@@ -240,9 +239,6 @@ int64_t TextSubtitleParser::seek_packet(void *opaque, int64_t offset, int whence
 /// rely on pkt->dts if you do not decompress the payload.
 int TextSubtitleParser::ReadNextSubtitle(void)
 {
-    // reset buffer
-    m_pkt->data = m_pkt->buf->data;
-    m_pkt->size = m_pkt->buf->size;
 
     int ret = av_read_frame(m_fmtCtx, m_pkt);
     if (ret < 0)