Fix crash when changing from blinking cursor to block cursor

Summary:
When using the new fancy escape codes to change the cursor,
we might hit an assert if the cursor was changed when the
cursor was hidden by blinking.

Test Plan:
run:

printf '\e[ q'

and then when the cursor is invisible run:

printf '\e[2 q'

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: konsole-devel, Konsole

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D17264

(cherry picked from commit e7a945741937b86fdbb020cf6213dabf944c113b)