Requesting send levels of non-existing channels is slow

[m-vo]

Using MIDI-over-TCP, when setting a send level of a target (e.g. mono aux 10) that is not available due to the current config (e.g. 6 mono aux buses configured), the response is extremely slow and an "Internal error" is logged. Repeatedly sending such messages slows down the whole communication.

IIRC, in these cases the mixrack also sends other unrelated data instead (for instanced interpreted as a group level).