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/Tcp4Dxe/Tcp4Input.c

@@ -710,6 +710,7 @@ TcpInput (
   Tcb     = NULL;
 
   Head    = (TCP_HEAD *) NetbufGetByte (Nbuf, 0, NULL);
+  ASSERT (Head != NULL);
   Len     = Nbuf->TotalSize - (Head->HeadLen << 2);
 
   if ((Head->HeadLen < 5) || (Len < 0) ||
@@ -1422,6 +1423,7 @@ TcpIcmpInput (
   BOOLEAN          IcmpErrNotify;
 
   Head = (TCP_HEAD *) NetbufGetByte (Nbuf, 0, NULL);
+  ASSERT (Head != NULL);
   Tcb = TcpLocateTcb (
           Head->DstPort,
           Dst,