BUG/MINOR: server: remove some incorrect free() calls on null elements

BUG/MINOR: server: remove some incorrect free() calls on null elements

In commit 6f4bfed3a ("MINOR: server: Add parser support for
set-proxy-v2-tlv-fmt") a few free() calls were made to an element on
error path when it was detected it was NULL. It doesn't have any
effect, however there was one case of use-after-free at the end of
srv_settings_cpy() that was caught by gcc due to attempting to free
the element after freeing its holder.

No backport is needed.