mpegts: prevent get_bits64() from reading nonsense at the end.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mpegts.c

@@ -693,6 +693,9 @@ static void new_pes_packet(PESContext *pes, AVPacket *pkt)
 static uint64_t get_bits64(GetBitContext *gb, int bits)
 {
     uint64_t ret = 0;
+
+    if (get_bits_left(gb) < bits)
+        return AV_NOPTS_VALUE;
     while (bits > 17) {
         ret <<= 17;
         ret |= get_bits(gb, 17);