Backend segfaults if the starting dir isn't contained in the top level folder

[daikema]

Describe the bug Unexpected segfault instead of just an error message when the starting dir isn't contained in the top level folder.

To Reproduce Steps to reproduce the behavior:

ubuntu@carta-dave:~$ carta_backend --version
4.1.0
ubuntu@carta-dave:~$ carta_backend --top_level_folder /mnt /home
[2024-06-14 10:27:36.154Z] [CARTA] [info] Writing to the log file: /home/ubuntu/.carta-beta/log/carta.log
[2024-06-14 10:27:36.155Z] [CARTA] [info] /usr/bin/carta_backend: Version 4.1.0
[2024-06-14 10:27:36.155Z] [CARTA] [critical] Starting /home must be a subdirectory of top level /mnt. Exiting carta.
Segmentation fault (core dumped)

Expected behavior An error message, but no segfault.

Platform info (please complete the following information):

• OS [e.g. macOS Monterey]: Ubuntu 22.04
• Browser [e.g. chrome, safari, electron app]: N/A
• Browser version [e.g. 22]: N/A
• Backend branch [e.g. dev, v3b2 release]: 4.1.0
• Frontend branch [e.g. dev, v3b2 release]: N/A

Additional context Picked dirs here at random just to duplicate on a more "normal" box an issue originally detected on a minimalist Ubuntu 22 pod.