avcodec/ffv1dec_template: Optimize common case in run mode

Fixes: Timeout (14sec -> 9sec)
Fixes: 13398/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFV1_fuzzer-5664106709778432

decode_line() becomes 1% faster for fate/vsynth2-ffv1.avi
for another fate sample there is a 0.5% speedup
the effect should be bigger for files with "flat" colored areas
the new faster branch is used in 97-100% of the cases in fate samples
compared to the older more complex (which i tested)

vsynth3-ffv1-v3-bgr0.avi had the lowest percentual useage of about 97%

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

libavcodec/ffv1dec_template.c

@@ -86,11 +86,19 @@ static av_always_inline int RENAME(decode_line)(FFV1Context *s, int w,
                         run_mode = 2;
                     }
                 }
+                if (sample[1][x - 1] == sample[0][x - 1]) {
+                    while (run_count > 1 && w-x > 1) {
+                        sample[1][x] = sample[0][x];
+                        x++;
+                        run_count--;
+                    }
+                } else {
                 while (run_count > 1 && w-x > 1) {
                     sample[1][x] = RENAME(predict)(sample[1] + x, sample[0] + x);
                     x++;
                     run_count--;
                 }
+                }
                 run_count--;
                 if (run_count < 0) {
                     run_mode  = 0;