Fixed RasPi builds of unit tests.

On RasPi with gcc 4.8.2, the asserts wouldn't compile when tests were enabled.
Enforced the assumption that ASSERT code is generated only in Debug builds.

src/ByteBuffer.cpp

@@ -135,7 +135,9 @@ bool cByteBuffer::Write(const void * a_Bytes, size_t a_Count)
 
 	// Store the current free space for a check after writing:
 	size_t CurFreeSpace = GetFreeSpace();
-	size_t CurReadableSpace = GetReadableSpace();
+	#ifdef _DEBUG
+		size_t CurReadableSpace = GetReadableSpace();
+	#endif
 	size_t WrittenBytes = 0;
 
 	if (CurFreeSpace < a_Count)