fuzz Failure: `Received counterparty stfu while having pending counterparty updates`

[tnull]

In #3804's CI we hit a fuzz failure of the chanmon_consistency target:

export HEX="ff2000000101b400f0ffff000001000000000000ffa1a1a1a1a100000001000000000000ffa1a1a1a1a1a1a1a100000000020000000f0000000000ffffffffff"

results in

ERROR 1 [lightning::ln::channelmanager : 12106] Got non-closing error: Received counterparty stfu while having pending counterparty updates