One of the calls to get the frame falloff was missing a
check for `use_multi_frame_falloff`.
This puts the conditions for when the multi frame falloff
needs to be calculated in the `get_frame_falloff` function.
That fixes the crash and makes a code a bit easier to read
(without ternary operator).
Pull Request: https://projects.blender.org/blender/blender/pulls/137259
62062c7915