NetworkPkg/TcpDxe: Check FragmentBuffer for NULL before use

NetworkPkg/TcpDxe: Check FragmentBuffer for NULL before use

According the Spec, the FragmentBuffers in FragmentTable are allocated
by the application when calling Receive() function. This patch is to
check whether the FragmentBuffer is valid or not.

Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wang Fan <fan.wang@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>