avcodec/sanm: Optimize fill_frame() with av_memcpy_backptr()

Fixes: Timeout (76 sec -> 24 sec)
Fixes: 15043/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SANM_fuzzer-5699856238116864

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

libavcodec/sanm.c

@@ -1358,8 +1358,10 @@ static int read_frame_header(SANMVideoContext *ctx, SANMFrameHeader *hdr)
 
 static void fill_frame(uint16_t *pbuf, int buf_size, uint16_t color)
 {
-    while (buf_size--)
+    if (buf_size--) {
         *pbuf++ = color;
+        av_memcpy_backptr((uint8_t*)pbuf, 2, 2*buf_size);
+    }
 }
 
 static int copy_output(SANMVideoContext *ctx, SANMFrameHeader *hdr)