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