NetworkPkg/Ip6Dxe: fix a bug in IP6 driver for IpSec protocol notify.

NetworkPkg/Ip6Dxe: fix a bug in IP6 driver for IpSec protocol notify.

The IP driver uses EfiCreateProtocolNotifyEvent() to register notify callback
function for IpSec protocol, but it didn't notice that the callback will always
be executed at least once, even the protocol wasn't in handle database.
As a result, the Ip6IpSecProcessPacket() will still always call LocateProtocol()
even the IpSec protocol is not installed, which will impact the network
performance.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>