buffer: combine checking range of sourceStart in `buf.copy`
Merging 2 checking range of sourceStart into 1. Plus, add test case to increase coverage if sourceStart is greater than length of source.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Merging 2 checking range of sourceStart into 1. Plus, add test case to increase coverage if sourceStart is greater than length of source.