Bug 30085: Don't fetch ReservesControlBranch twice

Bug 30085: Don't fetch ReservesControlBranch twice

We essentially copy the code from GetReservesControlBranch here, because we
also calculate 'branchfield'

Setting it to "" vs undef makes no difference in this code, so lets not fetch this
again later.

Rename the variable to make it clearer where the branchcode came from

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>