Fixed error handling in ngx_event_connect_peer().

Previously if ngx_add_event() failed a connection was freed two times (once
in the ngx_event_connect_peer(), and again by a caller) as pc->connection was
left set.  Fix is to always use ngx_close_connection() to close connection
properly and set pc->connection to NULL on errors.

Patch by Piotr Sikora.