MdeModulePkg/Mem: Initialize the variable MapMemory

MapMemory is not initialized by FindGuardedMemoryMap
or CoreInternalAllocatePages which calls MapMemory.
So we give a 0 to it.

Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Dandan Bi <dandan.bi@intel.com>

MdeModulePkg/Core/Dxe/Mem/HeapGuard.c

@@ -225,6 +225,8 @@ FindGuardedMemoryMap (
   UINTN                   BitsToUnitEnd;
   EFI_STATUS              Status;
 
+  MapMemory = 0;
+
   //
   // Adjust current map table depth according to the address to access
   //