Add more check to make code more safely.

Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ouyang Qian  <ouyang.qian@intel.com>

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13903 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Universal/Network/Ip4Dxe/Ip4Icmp.c

@@ -240,6 +240,7 @@ Ip4IcmpReplyEcho (
   // update is omitted.
   //
   Icmp                = (IP4_ICMP_QUERY_HEAD *) NetbufGetByte (Data, 0, NULL);
+  ASSERT (Icmp != NULL);
   Icmp->Head.Type     = ICMP_ECHO_REPLY;
   Icmp->Head.Checksum = 0;
   Icmp->Head.Checksum = (UINT16) (~NetblockChecksum ((UINT8 *) Icmp, Data->TotalSize));