Future-proof sort template against undefined behavior

Future-proof sort template against undefined behavior

Commit 176dffdf7 added a NULL array pointer check before performing
a qsort in order to prevent undefined behavior when passing NULL
pointer and zero length. To head off future degenerate cases, check
that there are at least two elements to sort before proceeding with
insertion sort. This has the added advantage of allowing us to remove
four equivalent checks that guarded against recursion/iteration.

There might be a tiny performance penalty from unproductive
recursions, but we can buy that back by increasing the insertion sort
threshold. That is left for future work.

Discussion: https://postgr.es/m/CANWCAZZWvds_35nXc4vXD-eBQa_=mxVtqZf-PM_ps=SD7ghhJg@mail.gmail.com