brstm: make sure an ADPC chunk was read for adpcm_thp

This fixes NULL pointer dereferencing.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
(cherry picked from commit d7d37c47

)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/brstm.c

@@ -260,6 +260,11 @@ static int read_packet(AVFormatContext *s, AVPacket *pkt)
     if (codec->codec_id == AV_CODEC_ID_ADPCM_THP) {
         uint8_t *dst;
 
+        if (!b->adpc) {
+            av_log(s, AV_LOG_ERROR, "adpcm_thp requires ADPC chunk, but none was found.\n");
+            return AVERROR_INVALIDDATA;
+        }
+
         if (av_new_packet(pkt, 8 + (32 + 4) * codec->channels + size) < 0)
             return AVERROR(ENOMEM);
         dst = pkt->data;