fs: Prevent the BFS version of get_wml_location() from escaping the d…

…ata dir

This is essentially a port to the BFS implementation of the fs API of
commits 314425a and
9f458d1, which were authored separately
from the BFS branch.

Note that this introduces another disparity in behavior between the BFS
and legacy implementations of this API, in that the BFS
get_wml_location() function will now always start at <data dir>/data/
instead of starting at <data dir> in obscure border cases. This is
expected to have no impact on anything at all for reasons discussed on
IRC.

src/filesystem_boost.cpp

@@ -1074,6 +1074,8 @@ std::string get_wml_location(const std::string &filename, const std::string &cur
 	if (!is_legal_file(filename))
 		return std::string();
 
+	assert(game_config::path.empty() == false);
+
 	path fpath(filename);
 	path result;
 
@@ -1082,7 +1084,13 @@ std::string get_wml_location(const std::string &filename, const std::string &cur
 		result /= get_user_data_path() / "data" / filename.substr(1);
 		DBG_FS << "  trying '" << result.string() << "'\n";
 	} else if (*fpath.begin() == ".") {
-		result /= path(current_dir) / filename;
+		if (!current_dir.empty()) {
+			result /= path(current_dir);
+		} else {
+			result /= path(game_config::path) / "data";
+		}
+
+		result /= filename;
 	} else if (!game_config::path.empty()) {
 		result /= path(game_config::path) / "data" / filename;
 	}