flacdec: change variable-length array to fixed length

pred_order can never exceed 32, so always allocating that amount is safe
and not very wasteful.

Originally committed as revision 19669 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/flacdec.c

@@ -358,7 +358,7 @@ static int decode_subframe_lpc(FLACContext *s, int channel, int pred_order)
 {
     int i, j;
     int coeff_prec, qlevel;
-    int coeffs[pred_order];
+    int coeffs[32];
     int32_t *decoded = s->decoded[channel];
 
     /* warm up samples */