This fixes:
> GEGL-WARNING: (../../src/gegl/gegl/buffer/gegl-tile.c:127):gegl_tile_dup: runtime check failed: (! src->damage)

Which happened when a GEGL operation was running and you canceled it in
the middle, say with the ESC key (easy to reproduce with long operations
such as "Color to Gray"). In such case, obviously don't try to copy the
unfinished operation result into the dest buffer.