Moved UserFile casting error back to sock_array_to_fd_set().

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.