Don't call preprocess_file() on an empty path if fonts.cfg can't be f…

…ound

get_wml_location() returns an empty string on failure, and there is
really no point in calling preprocess_file() on that -- it only results
in a confusing error message.

Replaces this sequence often found when Wesnoth can't resolve the path
to its data directory:

  20140612 23:37:05 error filesystem: Trying to open file with empty name.
  20140612 23:37:05 error config: Could not open file
  could not initialize fonts

... with the following one:

  20140612 23:38:52 error font: could not resolve path to fonts.cfg, file not found
  could not initialize fonts

... which should be a little bit more helpful in pointing users in the
right direction in the aforementioned case.

src/font.cpp

@@ -1258,7 +1258,13 @@ bool load_font_config()
 	//config when changing languages
 	config cfg;
 	try {
-		scoped_istream stream = preprocess_file(get_wml_location("hardwired/fonts.cfg"));
+		const std::string& cfg_path = get_wml_location("hardwired/fonts.cfg");
+		if(cfg_path.empty()) {
+			ERR_FT << "could not resolve path to fonts.cfg, file not found\n";
+			return false;
+		}
+
+		scoped_istream stream = preprocess_file(cfg_path);
 		read(cfg, *stream);
 	} catch(config::error &e) {
 		ERR_FT << "could not read fonts.cfg:\n"