fix crash introduced in previous patch

fix crash introduced in previous patch

In the previous patch,

-    my @subf = $field->subfields;
-    (defined @subf) or @subf = ();

is not equivalent to

+    my @subf = $field->subfields || ();

as, the results of $field->subields is interpeted in scalar
context, not list context, resutling in @subf containing
the number of subfields, not the subfield data itself, which
engenders the error

Can't use string ("1") as an ARRAY ref while "strict refs" in use

later on.  Changing the operator to 'or' instead of '||'
fixes this.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>