[22070] Fix memory leak: Missing regfree() after regcomp().

This fix is backported because it solves this bug:
Bug 681907 - Save operation is leaking seriously, eventually GnuCash uses 6.4 GB RAM

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/branches/2.4@23446 57a11ea4-9604-0410-9ed3-97b8803252fd