avcodec/pictordec: Only recalculate d when y changes in picmemset()

Fixes: Timeout (21->18sec)
Fixes: 14124/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PICTOR_fuzzer-5633887734071296

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

libavcodec/pictordec.c

@@ -68,10 +68,10 @@ static void picmemset(PicContext *s, AVFrame *frame, unsigned value, int run,
     int planel = *plane;
     value   <<= shift;
 
+    d = frame->data[0] + yl * frame->linesize[0];
     while (run > 0) {
         int j;
         for (j = 8-bits_per_plane; j >= 0; j -= bits_per_plane) {
-            d = frame->data[0] + yl * frame->linesize[0];
             d[xl] |= (value >> j) & mask;
             xl += 1;
             if (xl == s->width) {
@@ -85,6 +85,7 @@ static void picmemset(PicContext *s, AVFrame *frame, unsigned value, int run,
                    value <<= bits_per_plane;
                    mask  <<= bits_per_plane;
                 }
+                d = frame->data[0] + yl * frame->linesize[0];
             }
         }
         run--;