Bug 11269: fix incorrect use of MARC::Field->subfield()

Bug 11269: fix incorrect use of MARC::Field->subfield()

MARC::Record 2.0.6+ enables the warnings pragma, and as a
consequence, started logging cases where a routine in
C4::Search was calling MARC::Field->subfield() with an undef
subfield label.  This patch removes the log noise.

To test:
- Run prove -v t/db_dependent/Search.t
- There will be warnings about
  "Use of uninitialized value $code_wanted in string" in MARC::Field.
- Apply the patch.
- Those warnings are gone.

Signed-off-by: Liz Rea <liz@catalyst.net.nz>
Tests now pass

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 2f6f6d2b0c4d4e8a4ed5ab17fbbf66393f8a2c69)
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>