Fix deinstall of multiplexer when a default exists.

Fix deinstall of multiplexer when a default exists.

This copes with a somewhat subtle case where a default exists but the
last generator is removed. By inverting the checks, we can ensure we
only look at the cdr of the generator list when generators is actually a
pair.

The bug was originally exhibited by an error when removing the last
generator with remove-generic-procedure-generator on a generic procedure
with a default generator.