avformat/thp: Check av_get_packet() for failure not only for partial output

Fixes null pointer dereference
Fixes: signal_sigsegv_db2c1f_3108_cov_163322880_pikmin2_opening1_partial.thp

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

libavformat/thp.c

@@ -184,6 +184,8 @@ static int thp_read_packet(AVFormatContext *s,
         pkt->stream_index = thp->video_stream_index;
     } else {
         ret = av_get_packet(pb, pkt, thp->audiosize);
+        if (ret < 0)
+            return ret;
         if (ret != thp->audiosize) {
             av_free_packet(pkt);
             return AVERROR(EIO);