perl plugin: Disable plugin after fatal configuration errors.

Currently, the only such case is the failure to bootstrap the Perl interpreter
and Collectd module. This fixes a segfault that happened when trying to
configure Perl plugins in that case.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>