pool_alloc_vec() with too many elts.

[linas]

If form_match_list() would return a match list that more than 4090 of its elements would have a linkage (for both LHS and RHS), then a segfault would happen.

As I said before, this problem (of inability to allocate vectors that have more elements than num_elements) can relatively easily be solved in pool_alloc_vec(), so I can add that there. (A more drastic measure is not to cache such big lists but there is no need for that since the memory-pool fix is easy enough.)

Originally posted by @ampli in #1414 (comment)