codemaker: fix^2 invalid string index access

...originally the past-the-end checks in destination where always true, and thus
happend to work as intended for empty destionation, but
614e04019a672cdd61b86699d99250d80f169f95 broke that, so calling cppumaker w/o
-O was broken now.

Change-Id: I8d41dfe8d4c12e4a73a9782d4d5e7c9fa4d9df81
(cherry picked from commit d320760dc1c9d6501d84849219b4d9fc6c62b85b)
Reviewed-on: https://gerrit.libreoffice.org/7360
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>