Bug 12860: the export tools explodes if a field is not exported

Bug 12860: the export tools explodes if a field is not exported

To reproduce:
1/ Go on tools/export.pl
2/ Export some records and specify you don't want the fields 245 (the
whole field, do not specify a subfield).
3/ Export, BOOM
Software error:
Arguments must be MARC::Field object at /home/koha/src/tools/export.pl line 400.

Test plan:
Apply the patch and confirm the fields 245 are not exported and the
export works as expected.

Signed-off-by: Robin Sheat <robin@catalyst.net.nz>
Export fails to asplode with this patch

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>