Moved UserFile casting error back to sock_array_to_fd_set().

Summary: Moved UserFile's fd casting error back to sock_array_fd_set() function
to fix two errors in vfsstream's framework test -
vfsStreamWrapperSetOptionTestCase::setBlockingDoesNotWork and
vfsStreamWrapperSetOptionTestCase::removeBlockingDoesNotWork,
which is introduced in commit 17e0756.
Closes https://github.com/facebook/hhvm/pull/4401

Reviewed By: @JoelMarcey

Differential Revision: D1724140

Signature: t1:1724140:1418002744:d68376b4ccff6183c90b10f91f72fc98974f68d7