Fix bug in return of SelectingPoll.poll()

Selecting poll poll operation needs to return an empty array if no file descriptors are registered. You can't iterate over local variables (rl, wl) if they are not defined, so moved the original return statement into the else block and put an explicit 'return []' under the empty file descriptor sets condition.
Merge pull request #134 from dbotwinick/patch-1

Fix bug in return of SelectingPoll.poll()