Fix two minor defects:

1. CoreGetMemoryMap() returns unclean data
2. Zero length memory descriptor might be added to the memory map

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@4790 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Core/Dxe/Mem/Page.c

@@ -1432,7 +1432,7 @@ Returns:
   //
   // Build the map
   //
-  ZeroMem (MemoryMap, Size);
+  ZeroMem (MemoryMap, BufferSize);
   for (Link = gMemoryMap.ForwardLink; Link != &gMemoryMap; Link = Link->ForwardLink) {
     Entry = CR (Link, MEMORY_MAP, Link, MEMORY_MAP_SIGNATURE);
     ASSERT (Entry->VirtualStart == 0);