MdeModulePkg: Fix IPv4 double free

Fix a possible ASSERT after NBP finishes loading from a PXE
boot. Ip4Dxe driver calls Ip4Config2SetPolicy which calls
CloseEvent on the Dhcp4Event struct member.  After NBP is
downloaded, it then calls Ip4Config2CleanInstance which calls
CloseEvent again on Dhcp4Event.  This double free can cause an
ASSERT. When the event is closed, set the event pointer to NULL
so the Ip4 code won't call CloseEvent on it again.

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