Bug 9451: Quiet warnings and Plackify borrower_stats.pl

Bug 9451: Quiet warnings and Plackify borrower_stats.pl

There are several warnings and one non-Plack-compatible variable in
reports/borrower_stats.pl.

To test:
1) Apply patch
2) Use the borrower report wizard to create a report that includes an
   extended patron attribute.
3) Check the logs, and make sure that you don't see any of the
   following messages:
   Variable "$period" is not available at
     /home/jcamins/kohaclone/reports/borrowers_stats.pl line 265.
   Use of uninitialized value in hash element at
     /home/jcamins/kohaclone/reports/borrowers_stats.pl line 375.
   Use of uninitialized value in hash element at
     /home/jcamins/kohaclone/reports/borrowers_stats.pl line 376.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>