Wipe memory reliably in Memory::Zero

Memory::Zero() used a plain memset(), which the compiler is free to remove
as a dead store when the buffer is not read afterwards. It backs
BufferPtr::Erase() and BufferPtr::Zero(), which are used to clear key
material and other secrets, so an elided memset can leave secrets in memory.

Use burn() instead, the same volatile-write wipe already used by
Buffer::Erase(). Memory::Zero is not called on the bulk encryption/I/O path,
so this has no throughput impact.

Author: Damian Rickard

src/Platform/Memory.cpp

@@ -63,7 +63,13 @@ namespace VeraCrypt
 
 	void Memory::Zero (void *memory, size_t size)
 	{
-		memset (memory, 0, size);
+		// Use burn() rather than memset() so the write is not elided by the
+		// optimiser. Memory::Zero backs BufferPtr::Erase()/Zero(), which are
+		// used to wipe key material and other secrets; a plain memset() on a
+		// buffer that is not read afterwards can be removed as a dead store,
+		// leaving secrets in memory. burn() is the same volatile-write wipe
+		// already used by Buffer::Erase(). This is not on the bulk I/O path.
+		burn (memory, size);
 	}
 
 	void Memory::Free (void *memory)