fix by_url_belyi_search_url

[edgarcosta]

Prevents errors like:

Exception on /Belyi/foo-bar [GET]
Traceback (most recent call last):
  File "/home/sage/sage-9.7/local/var/lib/sage/venv-python3.10.5/lib/python3.10/site-packages/flask/app.py", line 2525, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/sage/sage-9.7/local/var/lib/sage/venv-python3.10.5/lib/python3.10/site-packages/flask/app.py", line 1822, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/sage/sage-9.7/local/var/lib/sage/venv-python3.10.5/lib/python3.10/site-packages/flask/app.py", line 1820, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/sage/sage-9.7/local/var/lib/sage/venv-python3.10.5/lib/python3.10/site-packages/flask/app.py", line 1796, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
  File "/home/lmfdb/lmfdb-git-dev/lmfdb/belyi/main.py", line 170, in by_url_belyi_search_url
    sigma1=sigma_spl[1],
IndexError: list index out of range
[2023-10-02 15:42:38 UTC] 500 error on URL https://beta.lmfdb.org/Belyi/foo-bar 

[AndrewVSutherland]

@SamSchiavone Are you planning to look at this?