perl: Don't call detach after failed perl_parse

Don't call "detach" callback in rlm_perl, if perl_parse of the Perl
module failed.

This fixes segfault when the module file cannot be read:

    Can't open perl script "/etc/raddb/mods-config/perl/example.pl": Permission denied
    rlm_perl: perl_parse failed: /etc/raddb/mods-config/perl/example.pl not found or has syntax errors.
    /etc/raddb/mods-enabled/perl[7]: Instantiation failed for module "perl"
    Segmentation fault
Merge pull request #802 from spbnick/dont_detach_without_perl_parse

perl: Don't call detach after failed perl_parse