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/Tcp4Misc.c

@@ -592,6 +592,7 @@ TcpFormatNetbuf (
 
   Seg       = TCPSEG_NETBUF (Nbuf);
   Head      = (TCP_HEAD *) NetbufGetByte (Nbuf, 0, NULL);
+  ASSERT (Head != NULL);
   Nbuf->Tcp = Head;
 
   Seg->Seq  = NTOHL (Head->Seq);