gtlspassword: Fix inverted assertion

The intention here was to assert that the length of the password fits
in a gssize. Passwords more than half the size of virtual memory are
probably excessive.

Fixes: a8b204ff

 "gtlspassword: Forbid very long TLS passwords"
Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 61bb52ec)