locale: Allow "" int_curr_Symbol (bug 22294)

locale: Allow "" int_curr_Symbol (bug 22294)

The builtin POSIX locale has "" as the international currency symbol,
but a non-builtin locale may not have such a blank int_curr_symbol.

Therefore to support non-builtin locales with similar "" int_curr_symbol
we adjust the LC_MONETARY parser to allow the normal 4-character
int_curr_symbol *and* the empty "" no symbol. Anything else remains
invalid.

Tested by building all the locales.  Tested also with a custom C.UTF-8
locale with "" for int_curr_symbol.

Signed-off-by: Carlos O'Donell <carlos@redhat.com>