Fix bug in CONTINUE statement for PL/pgSQL: when we continue a loop,

we need to be careful to reset rc to PLPGSQL_RC_OK, depending on how
the loop's logic is structured. If we continue a loop but it then
exits without executing the loop's body again, we want to return
PLPGSQL_RC_OK to our caller.  Enhance the regression tests to catch
this problem. Per report from Michael Fuhr.