Fix the len() method on LinearMap during popping

I found while using `LinearMap` that it would often get its size out of sync with the actual number of elements. I found out that during `pop` the invocations of `insert_opt_bucket` might increment the size even though nothing new was being inserted.

I tried to create a simple test case or two, but I was unable to recreate the exact sequence of operations which would lead to the bug being triggered.