Iterating through a segment with no mechanisms segfaults

[ramcdougal]

This works:

>>> from neuron import n
>>> soma = n.Section("soma")
>>> soma.insert(n.hh)
soma
>>> for mech in soma(0.5): break
... 
>>> mech
k_ion

But this segfaults:

>>> from neuron import n
>>> soma = n.Section("soma")
>>> for mech in soma(0.5): break
... 
zsh: segmentation fault  python

I'm running on an M2 mac, but I'm pretty sure this would also segfault on Linux based on #3792.

[ramcdougal]

Presumably solved by #3794 but need to check