MdeModulePkg/Core: fix an issue of potential NULL pointer access

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1286

This issue is introduced by bb685071c2.

The *MemorySpaceMap assigned with NULL (line 1710) value might be
accessed (line 1726/1730) without any sanity check. Although it won't
happen in practice because of line 1722, we still need to add check
against NULL to make static code analyzer happy.

1710  *MemorySpaceMap       = NULL;
....  ...
1722  if (DescriptorCount == *NumberOfDescriptors) {
....  ...
1726    Descriptor = *MemorySpaceMap;
....  ...
1730        BuildMemoryDescriptor (Descriptor, Entry);

Tests:
  Pass build and boot to shell.

Cc: Hao Wu <hao.a.wu@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>

MdeModulePkg/Core/Dxe/Gcd/Gcd.c

@@ -1719,7 +1719,7 @@ CoreGetMemorySpaceMap (
     // AllocatePool() called below has to be running outside the GCD lock.
     //
     DescriptorCount = CoreCountGcdMapEntry (&mGcdMemorySpaceMap);
-    if (DescriptorCount == *NumberOfDescriptors) {
+    if (DescriptorCount == *NumberOfDescriptors && *MemorySpaceMap != NULL) {
       //
       // Fill in the MemorySpaceMap if no memory space map change.
       //