VP3: Prevent stack corruption from an unset custom coding method.

Originally committed as revision 16978 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vp3.c

@@ -741,6 +741,8 @@ static int unpack_modes(Vp3DecodeContext *s, GetBitContext *gb)
 
         /* is it a custom coding scheme? */
         if (scheme == 0) {
+            for (i = 0; i < 8; i++)
+                custom_mode_alphabet[i] = MODE_INTER_NO_MV;
             for (i = 0; i < 8; i++)
                 custom_mode_alphabet[get_bits(gb, 3)] = i;
         }