curl_multi_wait: never return -1 in 'numfds'

Such a return value isn't documented but could still happen, and the
curl tool code checks for it. It would happen when the underlying
Curl_poll() function returns an error. Starting now we mask that error
as a user of curl_multi_wait() would have no way to handle it anyway.

Reported-by: Jay Satiro
Closes #707