afpd/volume: avoid read of unitialized pointer

Reported by Coverity as CID #980994.
libatalk: avoid double close in ad_openat

Partly revert 8852315e574879a9548c31f84c6eaea59f1db90f

Reported by Coverity as CID #980995.
afpd/file: fix check in deletefile

AD_RSRC_OPEN(ad) is defined as:

(((ad)->ad_reso_refcount) && (ad_reso_fileno(ad) >= 0))

So there's no way it can returns -1

Reported by coverity as CID #762087.
Merge pull request #19 from xrmx/coverity4

More issues reported by Coverity