Bug 27746: prevent regexp undefined warning when checking subfield contents ($oclc)

Some of the books have subfield "a" of field "035" absent
and it's possible to have such
(it doesn't says "Require" in the interface),
which causes "Use of uninitialized value $oclc in pattern match (m//)"
error.

Solved by screening regex with check for empty variables before the
match.

To reproduce:
    1) Edit existing book or create a new one with empty marc
subfield "a" of field "035"
    2) Use the search feature that will find it along with some other
books.
    3) Check /var/log/koha/kohadev/plack-intranet-error.log to find
"Use of uninitialized value $oclc in pattern match (m//)" error.
    4) Apply the patch.
    5) Repeat the search and check the logs again to ensure
that error didn't appear again.

Signed-off-by: David Nind <david@davidnind.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

C4/Koha.pm

@@ -671,7 +671,7 @@ sub GetNormalizedOCLCNumber {
         my @fields = $marcrecord->field('035');
         foreach my $field (@fields) {
             my $oclc = $field->subfield('a');
-            if ($oclc =~ /OCoLC/) {
+            if ($oclc && $oclc =~ /OCoLC/) {
                 $oclc =~ s/\(OCoLC\)//;
                 return $oclc;
             }