lavu/imgutils: Use FFABS() instead of abs() for ptrdiff_t.

Fixes a warning with clang:
libavutil/imgutils.c:314:16: warning: absolute value function 'abs'
given an argument of type 'ptrdiff_t' (aka 'long') but has
parameter of type 'int' which may cause truncation of value

libavutil/imgutils.c

@@ -311,8 +311,8 @@ static void image_copy_plane(uint8_t       *dst, ptrdiff_t dst_linesize,
 {
     if (!dst || !src)
         return;
-    av_assert0(abs(src_linesize) >= bytewidth);
-    av_assert0(abs(dst_linesize) >= bytewidth);
+    av_assert0(FFABS(src_linesize) >= bytewidth);
+    av_assert0(FFABS(dst_linesize) >= bytewidth);
     for (;height > 0; height--) {
         memcpy(dst, src, bytewidth);
         dst += dst_linesize;