If a freeze-frame retiming key was added to either a transition or
freeze-frame segment when the strip was selected, Blender would crash.
The issue is that `SEQ_retiming_add_key` returns `nullptr` if the start
key of the current segment is one of these types, and never bothers to
cancel the operation if this happens.
Instead, it only attempts to get a key at the current frame with
`SEQ_retiming_key_get_by_timeline_frame`. This is redundant, since there
are already checks to see if that key exists in `SEQ_retiming_add_key`.
Remove this code and move up the other `nullptr` check to fix the bug.
Pull Request: https://projects.blender.org/blender/blender/pulls/123981
8d8a84ffb4