[dvdplayer] fixes PGS subtitles

Statment  "if (m_Subtitle.pts != DVD_NOPTS_VALUE)" will never eval to false since ffmpeg sets pts to AV_NOPTS_VALUE. 

xbmc/cores/dvdplayer/DVDCodecs/Overlay/DVDOverlayCodecFFmpeg.cpp:
1. CDVDOverlayCodecFFmpeg::Decode ->
2. len = m_dllAvCodec.avcodec_decode_subtitle2(m_pCodecContext, &m_Subtitle, &gotsub, &avpkt); ->
3. lib/ffmpeg/libavcodec/utils.c:
static void avcodec_get_subtitle_defaults (AVSubtitle *sub)
{
    memset(sub, 0, sizeof(*sub));
    sub->pts = AV_NOPTS_VALUE;
}

DVD_NOPTS_VALUE  = 0xfff0000000000000
AV_NOPTS_VALUE  = 0x8000000000000000

./xbmc/cores/dvdplayer/DVDClock.h:
#define DVD_NOPTS_VALUE    (-1LL<<52) // should be possible to represent in both double and int64_t
./lib/ffmpeg/libavutil/avutil.h:
#define AV_NOPTS_VALUE          INT64_C(0x8000000000000000)