qdm2: fix out of stack array read.

The read value is not used when its outside the array.

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

libavcodec/qdm2.c

@@ -928,10 +928,10 @@ static int synthfilt_build_sb_samples (QDM2Context *q, GetBitContext *gb, int le
 
                 if (joined_stereo) {
                     float tmp[10][MPA_MAX_CHANNELS];
-
                     for (k = 0; k < run; k++) {
                         tmp[k][0] = samples[k];
-                        tmp[k][1] = (sign_bits[(j + k) / 8]) ? -samples[k] : samples[k];
+                        if ((j + k) < 128)
+                            tmp[k][1] = (sign_bits[(j + k) / 8]) ? -samples[k] : samples[k];
                     }
                     for (chs = 0; chs < q->nb_channels; chs++)
                         for (k = 0; k < run; k++)