fix crash when reading .po files

You may _never_ use the std:: file io functions
in wesnoth since they only support acii characters
on windows in filenames. We have fixed this many
times already but apparently the person that added
.po file support wasn't aware of this issue.

fixes #4965

src/gettext_boost.cpp

@@ -131,12 +131,11 @@ namespace
 				if(!filesystem::file_exists(path)) {
 					continue;
 				}
-				std::ifstream po_file;
-				po_file.exceptions(std::ios::badbit);
 				LOG_G << "Loading language file from " << path << '\n';
 				try {
-					po_file.open(path);
-					const po_catalog& cat = po_catalog::from_istream(po_file);
+					filesystem::scoped_istream po_file = filesystem::istream_file(path);
+					po_file->exceptions(std::ios::badbit);
+					const po_catalog& cat = po_catalog::from_istream(*po_file);
 					extra_messages_.emplace(get_base().domain(domain), cat);
 				} catch(const spirit_po::catalog_exception& e) {
 					throw_po_error(lang_name_long, domain, e.what());