erec: Dump locations' expressions only if set

Calling netlink_dump_expr() with a NULL pointer leads to segfault within
libnftnl. Internal ("fake") locations such as 'netlink_location' don't
have an expression assigned so expect this and skip the call. Simple
reproducer (list ruleset with netlink debugging as non-root):

| $ nft -d netlink list ruleset

Reported-by: François Rigault <frigo@amadeus.com>
Signed-off-by: Phil Sutter <phil@nwl.cc>

0 files changed, 0 insertions, 0 deletions