Replace an conditional assignment that was made obsolete by [d4c193f0b49f4950]

Replace an conditional assignment that was made obsolete by [d4c193f0b49f4950]
with an assert().  The conditional was added by [d6fd512f50513ab7] as
a fix for tickets [c36cdb4afd504dc1], [4051a7f931d9ba24], and
[d6fd512f50513ab7] which means now [d4c193f0b49f4950] is the correct fix
for those tickets.
that check-in

FossilOrigin-Name: 44b5524d522e749ad6bf76c94d754ff16c309c32439ec46802924663f64e8b09