mythburn.py: fix user defined chapter point creation

If the first user defined chapter point is not at 00:00:00 we add one there to
keep dvdauthor happy but there was one place where we didn't take the added
chapter point into account which was causing dvdauthor to fail with an invalid
jump to chapter point error.

(cherry picked from commit 989aba7aa5d0efa10c444cd722af1f4745b6154a)