Poll loop: fix read of unused fds[]

Poll loop: fix read of unused fds[]

Commit 29672515
 portability: Work around Apple's bizarrely deficient poll() implementation
introduced a read of unitialised data in fds[], because it failed to
honour i->nfds which is 0 on the first iteration.

The symptom is that secnet may, on the first iteration, die due to
mistakenly seeing POLLNVAL.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>