Avoid unneeded malloc/memcpy/free if there is no metadata on disk.

Submitted by:	Dmitry Luhtionov <dmitryluhtionov@gmail.com>
MFC after:	2 weeks
Merge remote-tracking branch 'freebsd/master' into hardened/current/master
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/unstable