BUG: fix NaT handling in _strided_to_strided_string_to_datetime

When converting an array of strings to datetime64, a "NaT" occurring
in the input would cause all subsequent values to be converted to NaT.
Merge pull request #4094 from cgwaldman/master

BUG: fix NaT handling in _strided_to_strided_string_to_datetime