desktop: Avoid useless warning for group header
Reported by Marek Černocký at:
<https://savannah.gnu.org/bugs/?44005>
* gettext-tools/src/read-desktop.c (desktop_lex): Fix
"invalid non-blank character" check after reading a group header.
* gettext-tools/tests/xgettext-desktop-1: Check "invalid non-blank
character" warning.
