threaded-resolver: never use NULL hints with getaddrinfo

The net effect of this bug as it appeared to users, would be that
libcurl would timeout in the connect phase.

When disabling IPv6 use but still using getaddrinfo, libcurl would
wrongly not init the "hints" struct field in init_thread_sync() which
would subsequently lead to a getaddrinfo() invoke with a zeroed hints
with ai_socktype set to 0 instead of SOCK_STREAM. This would lead to
different behaviors on different platforms but basically incorrect
output.

This code was introduced in 483ff1ca, released in curl 7.20.0.

This bug became a problem now due to the happy eyeballs code and how
libcurl now traverses the getaddrinfo() results differently.

Bug: http://curl.haxx.se/mail/lib-2014-01/0061.html
Reported-by: Fabian Frank
Debugged-by: Fabian Frank