Sync behavior of gdImageAutoCrop() with upstream

Sync behavior of gdImageAutoCrop() with upstream

Since cropping support has been added to our bundled libgd,
`gdImageAutoCrop` differs from upstream in that `GD_CROP_DEFAULT` falls
back on `GD_CROP_SIDES` if there is no transparent color in the image.
While this difference seem to be a useful improvement in our bundled
libgd, upstream has not yet signaled that there willing to back-port
it[1], so we revert it to stay in sync with upstream.

We also remove the additional NULL bailout at the end of the function,
which doesn't appear to be relevant any longer since bug 77198 has been
fixed.

[1] <https://github.com/libgd/libgd/issues/298>