soup_date_new_from_string: fix an out-of-bounds memory access

soup_date_new_from_string: fix an out-of-bounds memory access

soup_date_new_from_string() would sometimes read off the end of the
string, but tests/date didn't catch this even under valgrind, because
all of the strings were compile-time constants and they got laid out
in a way that never triggered a read of invalid memory. So tweak the
test to g_strdup each string before parsing it, which will let
valgrind notice if we read outside its bounds in the future.

https://bugzilla.gnome.org/show_bug.cgi?id=620288