[XFRM]: Fix existence lookup in xfrm_state_find

Use 'daddr' instead of &tmpl->id.daddr, since the latter
might be zero.  Also, only perform the lookup when
tmpl->id.spi is non-zero.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>