brstm: also allocate b->table in read_packet

This fixes NULL pointer dereferencing if the codec is forced to
adpcm_thp even though a different one was detected.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
(cherry picked from commit bcf4ee26)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/brstm.c

@@ -263,6 +263,10 @@ static int read_packet(AVFormatContext *s, AVPacket *pkt)
         if (!b->adpc) {
             av_log(s, AV_LOG_ERROR, "adpcm_thp requires ADPC chunk, but none was found.\n");
             return AVERROR_INVALIDDATA;
+        if (!b->table) {
+            b->table = av_mallocz(32 * codec->channels);
+            if (!b->table)
+                return AVERROR(ENOMEM);
         }
 
         if (av_new_packet(pkt, 8 + (32 + 4) * codec->channels + size) < 0)