core: Improve device memory and buffer queue handling

This commit makes several improvements to device memory management and buffer
queue handling:

DeviceMemoryManager:
- Add null pointer and size checks for ReadBlock
- Fill unmapped memory with a known pattern (0xCC) instead of zeros
- Add better error logging for invalid memory accesses
- Add null pointer check for source pointer in memcpy

BufferQueueProducer:
- Improve error handling in WaitForFreeSlotThenRelock
- Add proper abandoned state checking during wait conditions
- Clean up and simplify buffer scanning logic
- Improve logging messages with more descriptive information
- Remove redundant buffer count validation
- Fix potential infinite loop condition during wait

These changes improve stability and error handling while making the code more
maintainable and debuggable. The use of a known pattern for unmapped memory
helps identify uninitialized memory access issues.

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