Fixed a bug in ev_uinet_stop().  If ev_uinet_stop() was called on a
watcher that was being processed in the current event loop iteration,
and that watcher was the last remaining watcher for the corresponding
socket context, that socket context was not being removed from the
previous-pending list.

This could result in one or more of: upcalls being re-enabled for the
corresponding socket, access to and/or corruption of freed memory,
corruption of the pending list.

Also made a cosmetic change of a call to free() to ev_free().
Fix memory leak.