avcodec/scpr: Fix multiple runtime error: index 256 out of bounds for type 'unsigned int [256]'

Fixes: 1519/clusterfuzz-testcase-minimized-5286680976162816

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/scpr.c

@@ -261,6 +261,9 @@ static int decode_unit(SCPRContext *s, PixelModel *pixel, unsigned step, unsigne
             break;
         c++;
     }
+    if (x >= 16 || c >= 256) {
+        return AVERROR_INVALIDDATA;
+    }
 
     if ((ret = s->decode(gb, rc, cumfr, cnt_c, totfr)) < 0)
         return ret;