UefiCpuPkg/PiSmmCpuDxeSmm: Add check to avoid NULL ptr dereference

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c

@@ -1011,6 +1011,10 @@ GetUefiMemoryMap (
     }
   } while (Status == EFI_BUFFER_TOO_SMALL);
 
+  if (MemoryMap == NULL) {
+    return ;
+  }
+
   SortMemoryMap (MemoryMap, UefiMemoryMapSize, mUefiDescriptorSize);
   MergeMemoryMapForNotPresentEntry (MemoryMap, &UefiMemoryMapSize, mUefiDescriptorSize);
 
@@ -1080,6 +1084,10 @@ IsSmmCommBufferForbiddenAddress (
   UINTN                 MemoryMapEntryCount;
   UINTN                 Index;
 
+  if (mUefiMemoryMap == NULL) {
+    return FALSE;
+  }
+
   MemoryMap = mUefiMemoryMap;
   MemoryMapEntryCount = mUefiMemoryMapSize/mUefiDescriptorSize;
   for (Index = 0; Index < MemoryMapEntryCount; Index++) {