Wrong camera header timestamp due to wrong nanoseconds

[joshi-bharat]

The existing code produces an irregular camera header timestamp as

• header.nsec should only contain the nanoseconds
• image_ptr->GetTimeStamp() is uint64_t and header.nsec is uint32_t. This can cause integer overflow.

I have added assert to make sure that both timestamps are equal.

@tonybaltovski can you check this is important for visual SLAM algorithms that check the header stamp for synchronization.