Was introduced in 8708b9e081.
If these lookups are recursed to from (Chain)Context out-of-order,
it was possible that last_base > buffer->idx, in which case we
were attaching marks to a base after them... and an assertion
was failing fortunately.
Fixes https://oss-fuzz.com/testcase-detail/6377756666757120
Behdad Esfahbod