Bug 721791 - Segmentation fault when correcting invalid date

And greatly simplify gnc_split_register_get_date_help by just getting a GDate and running
g_date_strftime on it instead of messing around with Timespecs and g_localtime_r (twice!)
and all of that just to make a stupid string.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23706 57a11ea4-9604-0410-9ed3-97b8803252fd
Ensure struct tm is zeroed before passing it to gnc_tm_get_today_start

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23707 57a11ea4-9604-0410-9ed3-97b8803252fd