Fix a crash when accessing ._ files, bug #564

At least FreeBSD is picky in that it requires PROT_READ if we're going
to read a mmap()ed memory region mapped, only specifying PROT_WRITE
doesn't imply PROT_READ. Other OSen seem to handle this differently.

Also add a missing pointer reinitialisation to the buf pointer.

Reported-by: Sean Eric Fagan
Signed-off-by: Ralph Boehme <rb@sernet.de>
Merge branch 'branch-netatalk-3-1' of netafp.com:git/netatalk into branch-netatalk-3-1

Conflicts:
	NEWS