ss: fix NULL dereference when rendering without header

ss: fix NULL dereference when rendering without header

When ss is invoked with the no-header flag, if the query doesn't return
any result, render() is called with 'buffer' uninitialized. This
currently leads to a segfault. Ensure that buffer is initialized before
rendering.

The bug can be triggered with: ss -H sport = 100000

Signed-off-by: Jean-Philippe Brucker <jphilippe.brucker@gmail.com>
Acked-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>