Temporary sample format inconsistency between audio clip channels leading to failing assertion

[saintmatthieu]

Bug description

Debug only: Importing a mono wav file, duplicating the resulting track, making a stereo track out of them and then applying some effect asserts, where the invariant check is made that left and right channels have the same format.
Applying the effect replaces the clip's samples with a different format. After this is done for the left channel, there can be a temporary inconsistency, until the right channel samples are set as well, during which left channel has float and right has integer samples.

Steps to reproduce

1. On a debug build, import a mono mp3 file (I can provide one if needed)
2. Duplicate the resulting track
3. Join to stereo
4. Apply an effect (in my case compressor)

Expected behavior

Effect is applied.

Actual behavior

An assert is thrown.

Audacity Version

current master / alpha / nightly build

Operating system

Windows

Additional context

This is only a Debug problem so nothing critical for the release of 3.5. Yet I'm not sure how the code should be modified not to annoy developers and yet keep these sanity checks to maybe reveal more serious problems.
@Paul-Licameli maybe you'll have something to say about this.