No need to have pointers to mutables when not needed.

This does not cover everything at all, and is limited to one file for
the moment.

origin commit was:
https://github.com/mate-desktop/mate-panel/commit/b913762

The computed values are only used in one branch, so compute them there.

It's probably not necessary to perform the NULL check at all as the
only code paths that could make `list` NULL are already pretty
dramatically broken, but as we have a NULL check move everything
relevant inside it.

Mostly found by cppcheck.

origin commit was:
https://github.com/mate-desktop/mate-panel/commit/96c7ebc

This reverts commit 38e00280e9d17282717595a05411736a308804c0.

This reverts commit 7b1b00d5ddffc349ca117843bd0d015e9fb08a89.