When generating the display form of a heading that
happens to (invalidly) have a regular expression
metacharacter as a subfield label, do not crash.
An example of such a heading field is:
<datafield tag="650" ind1=" " ind2="0">
<subfield code="a">Dalziel, Andrew (Fictitious character</subfield>
<subfield code=")">xFiction.</subfield>
</datafield>
The error message associated with the crash is:
Unmatched ) in regex; marked by <-- HERE in m/) <-- HERE / at
/home/koha-pro/kohaclone/C4/Heading/MARC21.pm line 220.
Signed-off-by: Joshua Ferraro <jmf@liblime.com>
