Don't create new pad if a non-existant read-only pad is accessed

fixes #1848
Drop messages from unknown connections

fixes #1908
Merge pull request #1937 from ether/fix/non-existant-read-only-pads

Don't create new pad if a non-existant read-only pad is accessed
Merge pull request #1938 from ether/fix/unknown-connection-message

Drop messages from unknown connections
Merge branch 'develop' into release/1.2.12