Fix mapreduce() in a corner case

[nalimilan]

mapreduce_seq_impl() does not exist in Base for a long time (https://github.com/JuliaLang/julia/pull/16217), but this went unnoticed because since that code path is only used in very special conditions. Another consequence of this bug is that the return type of mapreduce_pairwise_impl_skipna() was inferred as Any.

Add a test which would have triggered the failure.

Also rename a variable to fix a type instability due to using it for two different things.