Fix zslUpdateScore() edge case.
Fix zslUpdateScore() edge case.

When the element new score is the same of prev/next node, the
lexicographical order kicks in, so we can safely update the node in
place only when the new score is strictly between the adjacent nodes
but never equal to one of them.

Technically speaking we could do extra checks to make sure that even if the
score is the same as one of the adjacent nodes, we can still update on
place, but this rarely happens, so probably not a good deal to make it
more complex.

Related to #5179.

JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis