Segfault in Kdenlive when adding crossfade on audio channel.

Hi there!

Kdenlive crashes quite reliably with the kdenlive project below, when adding in crossfades on the audio.

Crash_Test.kdenlive

Here is the GDB backtrace:

2023-08-30_Kdenlive-Crash.txt

And lastly, a video of how to induce the crash:

2023-08-30_15-43-36