Potential NPE when warningMessageCallback is null

[mrotteveel]

While running Sonarlint on master, it detected a potential NPE if the warningMessageCallback is null in AbstractWireOperations.consumePackets. It hasn't occurred in the wild because the method in question is currently only called in a case where the callback is not null, but this is easy to miss if that method might get called elsewhere due to future changes.

In addition, some code paths allow setting the defaultWarningMessageCallback of AbstractWireOperations to null and this should be disallowed (again those cases currently don't invoke the methods dereferencing warningMessageCallback or defaultWarningMessageCallback).

The fix should be backported to Jaybird 5 (no ticket will be created for Jaybird 6, as it will be committed as part of a larger code cleanup).