Never ever use uninitialised descriptors if pipe() fails

Never ever use uninitialised descriptors if pipe() fails
(because of maximum open count fd exceeded or so).

Better return busy message, so even clients fail in more controlled way.

Without patch,  clvmd crashes with various strange backtraces
if under heavy load of connection requests.

https://bugzilla.redhat.com/show_bug.cgi?id=698393
https://bugzilla.redhat.com/show_bug.cgi?id=697945

(It still can OOM but that's different issue.)