bug 2315: no crash if subfield code is a metacharacter

bug 2315: no crash if subfield code is a metacharacter

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>