net: Zero sockaddr_in in parse_host_port()

We don't currently zero-initialize the 'struct sockaddr_in' that
parse_host_port() fills in, so any fields we don't explicitly
initialize might be left as random garbage.  POSIX states that
implementations may define extensions in sockaddr_in, and that those
extensions must not trigger if zero-initialized.  So not zero
initializing might result in inadvertently triggering an impdef
extension.

memset() the sockaddr_in before we start to fill it in.

Fixes: Coverity CID 1005338
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20210813150506.7768-2-peter.maydell@linaro.org