Fix uninitalized values in Waveshaper

[falkTX]

Fixes #992

1st change is needed for the case where plugin UI runs while Rack/Cardinal is bypassed which makes this:

ddb = module->modulationAssistant.values[Waveshaper::DRIVE][ch];

read uninitialized values (they are only set during processing)

2nd change is more obvious, fixes the use of displayPolyChannel which does not have a value set.
This also matches what https://cplusplus.com/reference/atomic/atomic/atomic/ says

(1) default constructor
Leaves the atomic object in an uninitialized state.

which I didn't know it was a thing, I thought std::atomic types would initialize to 0...

[baconpaul]

Thanks!

On the second one {} would also work since that would use the default constructor for T but nothing is indeed an error and {0} is most appropriate.