shorten: Extend fixed_coeffs to properly support pred_order 0

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/shorten.c

@@ -273,7 +273,8 @@ static void output_buffer(int16_t **samples, int nchan, int blocksize,
     }
 }
 
-static const int fixed_coeffs[3][3] = {
+static const int fixed_coeffs[][3] = {
+    { 0,  0,  0 },
     { 1,  0,  0 },
     { 2, -1,  0 },
     { 3, -3,  1 }
@@ -302,7 +303,12 @@ static int decode_subframe_lpc(ShortenContext *s, int command, int channel,
     } else {
         /* fixed LPC coeffs */
         pred_order = command;
-        coeffs     = fixed_coeffs[pred_order - 1];
+        if (pred_order > FF_ARRAY_ELEMS(fixed_coeffs)) {
+            av_log(s->avctx, AV_LOG_ERROR, "invalid pred_order %d\n",
+                   pred_order);
+            return AVERROR_INVALIDDATA;
+        }
+        coeffs     = fixed_coeffs[pred_order];
         qshift     = 0;
     }