bug 3159: allow editing child patron that has no valid guarantor

bug 3159: allow editing child patron that has no valid guarantor

If a patron with a child patron category is not linked
to a guarantor (or a valid one), it was not possible to edit
the child patron's record.  When attempt, no changes were saved
and the following error is recorded in the Apache error log:

memberentry.pl: DBD::mysql::st execute failed: Cannot add or
update a child row: a foreign key constraint
fails (`gmc_koha/borrowers`, CONSTRAINT `borrowers_ibfk_2`
FOREIGN KEY (`branchcode`) REFERENCES `branches` (`branchcode`)) ...

This patch fixes this bug, allowing a child patron record to be
edited even if it has no guarantor.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>