memtrace: Update nBlocks when pointer changes in MemoryShadow::cover

During each ::Map different pointers could be returned by DirectX.
Thus for the same resource with the same subresource having the same size
pointer may change between calls of MemoryShadow::cover. With the pointer
change number of blocks also may change, which may later cause memory
access violation in MemoryShadow::update.

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Signed-off-by: Andrii Kryvytskyi <andrii.o.kryvytskyi@globallogic.com>