afpd/volume: don't leak vol_mname in afp_openvol

Reported as Coverity by CID #201201
afpd/hash: avoid use after free in main in case of OOM

Reported by Coverity as CID #968574, #968575.
afpd/fork: avoid use after free on of_closefork error

Reported by Coverity as CID #201311.
afpd/fce_api: avoid null pointer dereference

Move path NULL check before it is actually referenced.

Reported by Coverity as CID #968570.
Merge pull request #15 from xrmx/coverity1

Fix some issues reported by Coverity scan