Bug 6100: request.pl should check maxreserves exists

Bug 6100: request.pl should check maxreserves exists

Adds a check to confirm that the maxreserves syspref actually has a value before
using it. Also fixes an off-by-one error in the maxreserves calculation that
would allow librarians to place maxreserves + 1 holds for patrons.

Signed-off-by: Ian Walls <ian.walls@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
(cherry picked from commit 8b582d863debf1a60c69560b1cb797a9931bdcf8)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>