Fix a file descriptor leak in mdXhl.c (which is used by numerous hashing
algorithms.

CID: 1305669,1305611,1305663,1305603,1305584,1305639,1346865,1305601
Differential Revision: https://reviews.freebsd.org/D4732
Reviewed by: allanjude, delphij
MFC after: 2 weeks
Sponsored by: Juniper Networks