Bug 29204: (QA follow-up) Fix variable name

Bug 29204: (QA follow-up) Fix variable name

The patch "Bug 29204: Fix reports/issues_stats.pl for MySQL 8"
accidentally changed the variable name from $strsth2 to $strsth

Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 5d2f51f9ad9ec5f46cc7a5ef2264ad9c5a188199)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>