psql: Fix memory leak with repeated calls of \bind

psql: Fix memory leak with repeated calls of \bind

Calling \bind repeatedly would cause the memory allocated for the list
of bind parameters to be leaked after each call, as the list is reset
when beginning a single call.

This issue is fixed by making the cleanup of the bind parameter list
more aggressive, refactoring it into a single routine called after
processing a query and before running an individual \bind.

HEAD required more surgery and has been fixed by 87eeadaea143.  Issue
introduced by 5b66de3433e2.

Reported-by: Anthonin Bonnefoy
Discussion: https://postgr.es/m/2e5b89af-a351-ff0a-000c-037ac28314ab@gmail.com
Backpatch-through: 16