Allow uid of -1 for a PolkitUnixProcess

Commit 2cb40c4d changed PolkitUnixUser, PolkitUnixGroup, and
PolkitUnixProcess to allow negative values for their uid/gid properties,
since these are values above INT_MAX which wrap around but are still
valid, with the exception of -1 which is not valid. However,
PolkitUnixProcess allows a uid of -1 to be passed to
polkit_unix_process_new_for_owner() which means polkit is expected to
figure out the uid on its own (this happens in the _constructed
function). So this commit removes the check in
polkit_unix_process_set_property() so that new_for_owner() can be used
as documented without producing a critical error message.

This does not affect the protection against CVE-2018-19788 which is
based on creating a user with a UID up to but not including 4294967295
(-1).