amcheck: Fixes for right page check during unique constraint check

amcheck: Fixes for right page check during unique constraint check

 * Don't forget to pfree() the right page when it's to be ignored.
 * Report error on unexpected non-leaf right page even if this page is not
   to be ignored.  This restores the logic which was unintendedly changed
   in 97e5b0026f.

Reported-by: Pavel Borisov