avcodec/videodsp_template: Fix overflow of addition

Fixes: addition of unsigned offset to 0x7f56fc26a9b6 overflowed to 0x7f56fc26a8be*
Fixes: clusterfuzz-testcase-minimized-mediasource_MP4_AVC1_pipeline_integration_fuzzer-4917949056679936

Reported-by: Matt Wolenetz <wolenetz@google.com>
Reviewed-by: Matt Wolenetz <wolenetz@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/videodsp_template.c

@@ -44,7 +44,8 @@ void FUNC(ff_emulated_edge_mc)(uint8_t *buf, const uint8_t *src,
         src_y = 1 - block_h;
     }
     if (src_x >= w) {
-        src  += (w - 1 - src_x) * sizeof(pixel);
+        // The subtracted expression has an unsigned type and must thus not be negative
+        src  -= (1 + src_x - w) * sizeof(pixel);
         src_x = w - 1;
     } else if (src_x <= -block_w) {
         src  += (1 - block_w - src_x) * sizeof(pixel);