BUG/MINOR: http_fetch: fix possible uninit sockaddr in fetch_url_ip/port

BUG/MINOR: http_fetch: fix possible uninit sockaddr in fetch_url_ip/port

Check the return value of url2sa in smp_fetch_url_ip/port. If negative,
the address result is uninitialized and the sample fetch is aborted.
Also, the sockaddr is prelimiary zero'ed before calling url2sa to ensure
that it is not used by upper functions even if the sample returns 0.

Without the check, the value returned by the url_ip/url_port fetches is
unspecified. This can be triggered with the following curl :
$ curl -iv --request-target "xxx://127.0.0.1:20080/" http://127.0.0.1:20080/

This should be backported to all stable branches. However, note that
between the 1.8 and 2.0, the targetted functions have been extracted
from proto_http.c to http_fetch.c.

This should fix in part coverity report from the github issue #1244.