#477 psrzap crashes when ran on files f-scrunched with pam

[Emma Carli]

Hello,
When using psrzap on archives that have been previously f-scrunched with pam -m, this error sometimes comes up:
psrzap error:

Error::stack 
    Integration::set_weight 
Error::InvalidRange 
Error::message 
    ichan=-1 nchan=416 

This doesn't seem to happen when ran on original frequency resolution archives. This happens regularly to two users on various installs.
Thanks in advance for your time.
Best
Emma