Avoid DEBUG_CLEAR_MEMORY clearing MemoryMap internal structure.

Avoid DEBUG_CLEAR_MEMORY clearing MemoryMap internal structure.
In CoreFreePages(), the following sequence might break the MemoryMap internal structure:
CoreConvertPages() -> CoreFreeMemoryMapStack() -> AllocateMemoryMapEntry() -> CoreAllocatePoolPages() -> DEBUG_CLEAR_MEMORY()

CoreConvertPages() will call CoreFreeMemoryMapStack() after it adds the freed memory range, so the latter might use the just freed memory range when calling AllocateMemoryMapEntry(). But CoreFreePages() will call DEBUG_CLEAR_MEMORY() after CoreConvertPages(). This might clear up the memory map entry structure.

The fix calls DEBUG_CLEAR_MEMORY() just after freed memory range is added in CoreConvertPages(), which is safe.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@10335 6f19259b-4bc3-4df7-8a09-765794883524