MdeModulePkg: Add ASSERT to make sure pointer 'MemoryMap' is not NULL.
MdeModulePkg: Add ASSERT to make sure pointer 'MemoryMap' is not NULL.

If 'CoreGetMemoryMap' return success the 'MemoryMap' shouldn't be NULL.
Add code to make sure this.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <a85ffd5e6451a83645d9ceae55f1cb6cdf90c130@intel.com>
Reviewed-by: Yao Jiewen <3e40e3cc7483ad7ef780b74eea1ba30cb8bc9bdf@intel.com>

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