Fix crash in AICurlPrivate::curlthread::PollSet::next

Introduced in
https://github.com/AlericInglewood/SingularityViewer/commit/6dcda3595e5ff176ec05c7156a68413390636ed1
(Add recovery for randomly closed socket desciptors.)

By copying CurlSocketInfo* into mCopiedFileDescriptors, it was possible
that we accessed a deleted CurlSocketInfo for it's filedescriptor,
returning a random value, which, when passed to FD_ISSET could cause a
SIGSEGV.

I reverted this change and now store the literal socket descriptors in
mCopiedFileDescriptors again. It is not a problem when libcurl deletes
the corresponding CurlSocketInfo (through a callback to
MultiHandle::socket_callback) in that case, because the fd_set we test
against isn't updated as a result of that, not until we're down with
mCopiedFileDescriptors (at the re-entry of select()).