Check that the dirlist path string specification does not cause
overflow and is fully contained in the hints file.
Check that the dirlist string is nul-terminated.
Make 'hdr' static variable, so that hdr.dirlistlen is available when
hints cached value is used on next function calls. Reset hdr.dirlistlen
to zero if error was detected, so that allocations use reasonable size.
Use 'hints', and not 'p' in the body, since p is only initialized on the
first call.
Reported and reviewed by: truckman (previous version)
Sponsored by: The FreeBSD Foundation
CIDs: 1006503, 1006504, 1006676, 1008488, 1007263
MFC after: 2 weeks
95a437124b