MdeModulePkg: Fix conditionally uninitialized variables

Fixes CodeQL alerts for CWE-457:
https://cwe.mitre.org/data/definitions/457.html

Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Erich McMillan <emcmillan@microsoft.com>
Cc: Guomin Jiang <guomin.jiang@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Michael Kubacki <mikuback@linux.microsoft.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Co-authored-by: Erich McMillan <emcmillan@microsoft.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Oliver Smith-Denny <osd@smith-denny.com>

MdeModulePkg/Core/Dxe/Mem/Page.c

@@ -449,14 +449,15 @@ PromoteMemoryResource (
     //
     Promoted = PromoteGuardedFreePages (&StartAddress, &EndAddress);
     if (Promoted) {
-      CoreGetMemorySpaceDescriptor (StartAddress, &Descriptor);
-      CoreAddRange (
-        EfiConventionalMemory,
-        StartAddress,
-        EndAddress,
-        Descriptor.Capabilities & ~(EFI_MEMORY_PRESENT | EFI_MEMORY_INITIALIZED |
-                                    EFI_MEMORY_TESTED | EFI_MEMORY_RUNTIME)
-        );
+      if (!EFI_ERROR (CoreGetMemorySpaceDescriptor (StartAddress, &Descriptor))) {
+        CoreAddRange (
+          EfiConventionalMemory,
+          StartAddress,
+          EndAddress,
+          Descriptor.Capabilities & ~(EFI_MEMORY_PRESENT | EFI_MEMORY_INITIALIZED |
+                                      EFI_MEMORY_TESTED | EFI_MEMORY_RUNTIME)
+          );
+      }
     }
   }