xfrm: policy: fix netlink/pf_key policy lookups

Colin Ian King says:
 Static analysis with CoverityScan found a potential issue [..]
 It seems that pointer pol is set to NULL and then a check to see if it
 is non-null is used to set pol to tmp; howeverm this check is always
 going to be false because pol is always NULL.

Fix this and update test script to catch this.  Updated script only:
./xfrm_policy.sh ; echo $?
RTNETLINK answers: No such file or directory
FAIL: ip -net ns3 xfrm policy get src 10.0.1.0/24 dst 10.0.2.0/24 dir out
RTNETLINK answers: No such file or directory
[..]
PASS: policy before exception matches
PASS: ping to .254 bypassed ipsec tunnel
PASS: direct policy matches
PASS: policy matches
1

Fixes: 6be3b0db

 ("xfrm: policy: add inexact policy search tree infrastructure")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>