[MirrorOrch]: Init the next hop ip with 0 instead of default construc…

…tor (sonic-net#953)

    Otherwise, in the getgetNeighborInfo function, it will check whether
    the nexthop is zero, but it will return false according to
    IpAddress::isZero(), then it will try to query the
    neighbor orch about this invalid ip address.

Signed-off-by: Richard Wu <wutong23@baidu.com>

orchagent/mirrororch.cpp

@@ -786,7 +786,7 @@ void MirrorOrch::updateNextHop(const NextHopUpdate& update)
         }
         else
         {
-            session.nexthopInfo.nexthop = IpAddress();
+            session.nexthopInfo.nexthop = IpAddress(0);
         }
 
         // Resolve the neighbor of the new next hop