avcodec/ansi: fix integer overflow

Fixes out of array read
Fixes: 5f9698e86d92f19bb08d54ff0d57027f-signal_sigsegv_b30756_3795_cov_2693691257_ansi256.ans
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ansi.c

@@ -420,7 +420,7 @@ static int decode_frame(AVCodecContext *avctx,
             switch(buf[0]) {
             case '0': case '1': case '2': case '3': case '4':
             case '5': case '6': case '7': case '8': case '9':
-                if (s->nb_args < MAX_NB_ARGS)
+                if (s->nb_args < MAX_NB_ARGS && s->args[s->nb_args] < 6553)
                     s->args[s->nb_args] = FFMAX(s->args[s->nb_args], 0) * 10 + buf[0] - '0';
                 break;
             case ';':