Update connection/connection.c

according to "man epoll_ctl":
BUGS
       In kernel versions before 2.6.9, the EPOLL_CTL_DEL operation required a non-NULL pointer in event, even though this argument is ignored.  Since kernel  2.6.9,
       event can be specified as NULL when using EPOLL_CTL_DEL.
Merge pull request #10 from lx1988cyk/patch-1

Update connection/connection.c