MNT: Add additional assert to check
MNT: Add additional assert to check

If there are no log messages, before this change you just get an
IndexError. Now you get a failed assert and it's quicker to see what's
going on.

jrleeman/MetPy,ahaberlie/MetPy,Unidata/MetPy,ShawnMurd/MetPy,jrleeman/MetPy,dopplershift/MetPy,ahaberlie/MetPy,dopplershift/MetPy,Unidata/MetPy