MAINT: initialize strides for ufunc.at

avoids uninitialized use errors from valgrind, value of strides is
irrelevant as count is always 1.
BUG: fix uninitialized use in _strided_to_strided_string_to_datetime

dt == NPY_DATETIME_NAT is used as error condition, so initialize it to
something else.
Merge pull request #4051 from juliantaylor/valgrind

fix two uninitialized uses reported with valgrind