Bug 23822: Fix deletion of patrons with credit

Bug 23822: Fix deletion of patrons with credit

There are bugs in both master and 19.05, but different.

Anyway we should have this check to make sure a negative value will have
the same behavior: trigger the confirmation message (instead of a blank
page).

If we want to reject the deletion of a patron with credit we should
handle it on a separate bug report (behavior change)

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit d23e50f0b239603a05ddbb62591e80657f5f6cae)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>