avcodec/videodsp_template: Adjust pointers to avoid undefined pointer things

Fixes: subtraction of unsigned offset from 0xf6602770 overflowed to 0xf6638c80
Fixes: 48567/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_THEORA_fuzzer-495074400600064

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

libavcodec/videodsp_template.c

@@ -64,7 +64,7 @@ void FUNC(ff_emulated_edge_mc)(uint8_t *buf, const uint8_t *src,
     av_assert2(start_x < end_x && block_w);
 
     w    = end_x - start_x;
-    src += start_y * src_linesize + start_x * sizeof(pixel);
+    src += start_y * src_linesize + start_x * (ptrdiff_t)sizeof(pixel);
     buf += start_x * sizeof(pixel);
 
     // top
@@ -87,7 +87,7 @@ void FUNC(ff_emulated_edge_mc)(uint8_t *buf, const uint8_t *src,
         buf += buf_linesize;
     }
 
-    buf -= block_h * buf_linesize + start_x * sizeof(pixel);
+    buf -= block_h * buf_linesize + start_x * (ptrdiff_t)sizeof(pixel);
     while (block_h--) {
         pixel *bufp = (pixel *) buf;