fs: Use game data path to resolve ./ in the absence of a current_dir

Fixes a file content disclosure bug (#22042) affecting functionality
relying on the get_wml_location() function and not passing a non-empty
value for the current_dir parameter.

See <https://gna.org/bugs/?22042> for details.

This is a candidate for the 1.10 and 1.12 branches.
fs: Assert that game_config::path is not empty in get_wml_location()

Let's see if this breaks some absurd border case I couldn't think of.
Quick testing revealed that early initialization, campaignd, and
wesnothd at least did not fail the assertion check.