dir.c: use st_add3() for allocation size

dir.c: use st_add3() for allocation size

When preparing a manufactured dirent instance, we add a length of
path to the size of struct to decide how many bytes to allocate.
Make sure this addition does not wrap-around to cause us
underallocate.

Suggested-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>