Bug 17294: Make reserves_stats.pl plack safe

Bug 17294: Make reserves_stats.pl plack safe

Lot of errors are raised by this script under plack because of var scope
issues:

Variable "$ccodes" is not available at
/home/koha/src/reports/reserves_stats.pl line 337.
Variable "$locations" is not available at
/home/koha/src/reports/reserves_stats.pl line 338.
Variable "$itemtypes" is not available at
/home/koha/src/reports/reserves_stats.pl line 339.
Variable "$Bsort1" is not available at
/home/koha/src/reports/reserves_stats.pl line 344.
Variable "$Bsort2" is not available at
/home/koha/src/reports/reserves_stats.pl line 350.
Variable "@patron_categories" is not available at
/home/koha/src/reports/reserves_stats.pl line 356.

The lazy way to fix that is that reinit them from the subroutines they
are used.

Test plan:
Execute the report, after this patch applied you won't see the error in
the log.

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Katrin Fischer  <katrin.fischer@bsz-bw.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>