MdeModulePkg/DxeCore: Please static checker for false report

MdeModulePkg/DxeCore: Please static checker for false report

After commit 57df17fe26, some static check reports suspicious NULL pointer
deference at line:

  Entry->MachineType = Entry->Emulator->MachineType;
                       ^^^^^^^^^^^^^^^

within function PeCoffEmuProtocolNotify().

However, 'Entry->Emulator' is guaranteed to have a non-NULL value when
previous call to the CoreHandleProtocol() returns EFI_SUCCESS.

This commit will re-write the return status check for CoreHandleProtocol()
to add explicit NULL pointer check for protocol instance pointer.

Cc: Jian J Wang <jian.j.wang@intel.com>
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>