Well not really fixed, more just added an assert to make sure
lfsr_rbyd_lookup is not called with tag 0. Because our alt tags only
encode less-than-or-equal and greater-than, which can be flipped
trivially, it's not possible to encode removal of tag 0 during deletes.
Fortunately, this tag should already not exist for other pragmatic
reasons, it was just used as the initial value for traversals, where it
could cause this bug.
Christopher Haster
745b89d02b