xgettext: In language JavaScript, avoid a crash for some non-ASCII input.
* gettext-tools/src/xg-encoding.c (non_ascii_error_message,
non_utf8_error_message): Handle all possible lexical_context_ty values.
* gettext-tools/tests/xgettext-javascript-8: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.