avoid valgrind false positive by zeroing epoll_event

Not sure what your feeling on valgrind is, and whether you feel it's worth it to add a little bit of code to avoid valgrind false positives.  If not, that's fine, but if you do, this one-line memset avoids this valgrind false positive I was running into:

    ==26101== Syscall param epoll_ctl(event) points to uninitialised byte(s)
    ==26101==    at 0x5FC327A: epoll_ctl (syscall-template.S:82)
    ==26101==    by 0x53EF8DA: epoll_init (epoll.c:187)
    ==26101==    by 0x53E73F5: event_base_new_with_config (event.c:633)
    ==26101==    by 0x53E7524: event_base_new (event.c:468)
    ==26101==    by 0x409093: main (http-example-client.c:174)
    ==26101==  Address 0x7feffffb8 is on thread 1's stack
    ==26101==  Uninitialised value was created by a stack allocation
    ==26101==    at 0x53EF7E0: epoll_init (epoll.c:130)