These were callers which explicitly specified the string length to
g_utf8_validate(), when it couldn’t be negative, and hence should be
able to unconditionally benefit from the increased string handling
length.

At least one call site would have previously silently changed behaviour
if called with strings longer than G_MAXSSIZE in length.

Another call site was passing strlen(string) to g_utf8_validate(), which
seems pointless: just pass -1 instead, and let g_utf8_validate()
calculate the string length. Its behaviour on embedded nul bytes
wouldn’t change, as strlen() stops at the first one.

Signed-off-by: Philip Withnall <withnall@endlessm.com>