qdm2: check that coding_method is valid before using it.

Fixes out of array reads.

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

libavcodec/qdm2.c

@@ -795,6 +795,11 @@ static int synthfilt_build_sb_samples (QDM2Context *q, GetBitContext *gb, int le
                 for (j = 0; j < 16; j++)
                     sign_bits[j] = get_bits1 (gb);
 
+            if (q->coding_method[0][sb][0] <= 0) {
+                av_log(NULL, AV_LOG_ERROR, "coding method invalid\n");
+                return AVERROR_INVALIDDATA;
+            }
+
             for (j = 0; j < 64; j++)
                 if (q->coding_method[1][sb][j] > q->coding_method[0][sb][j])
                     q->coding_method[0][sb][j] = q->coding_method[1][sb][j];