selftests: mptcp: Initialize variables to quiet gcc 12 warnings

commit fd37c2ec upstream.

In a few MPTCP selftest tools, gcc 12 complains that the 'sock' variable
might be used uninitialized. This is a false positive because the only
code path that could lead to uninitialized access is where getaddrinfo()
fails, but the local xgetaddrinfo() wrapper exits if such a failure
occurs.

Initialize the 'sock' variable anyway to allow the tools to build with
gcc 12.

Fixes: 048d19d4 ("mptcp: add basic kselftest for mptcp")
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ mptcp_inq.c and mptcp_sockopt.c are not in this version. The fix can
  still be applied in mptcp_connect.c without conflicts. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>