chase: check the result is a directory or regular file only when the resolved path...

chase: check the result is a directory or regular file only when the resolved path exists

Otherwise, if it is called with CHASE_NONEXISTENT, when we call
stat_verify_directory()/_regular() the struct stat is for one of the
parent directory, rather than for the result path.

With this change, we can safely specify CHASE_MUST_BE_DIRECTORY/REGULAR
with CHASE_NONEXISTENT.

More importantly, chaseat() internally sets CHASE_MUST_BE_DIRECTORY when
the input path ends with "/", "/,", "/..". Hence, without this change,
we cannot specify CHASE_NONEXISTENT safely.

Follow-up for 90b9f7a07e6f57825f416f6ce2db0a9f2086754b.