Fix hypothetical bug in heap backward scans

 Fix hypothetical bug in heap backward scans
 
 Both heapgettup() and heapgettup_pagemode() incorrectly set the first page
 to scan in a backward scan in which the number of pages to scan was
 specified by heap_setscanlimits().  The code incorrectly started the scan
 at the end of the relation when startBlk was 0, or otherwise at
 startBlk - 1, neither of which is correct when only scanning a subset of
 pages.
 
 The fix here checks if heap_setscanlimits() has changed the number of
 pages to scan and if so we set the first page to scan as the final page in
 the specified range during backward scans.
 
 Proper adjustment of this code was forgotten when heap_setscanlimits() was
 added in 7516f5259 back in 9.5.  However, practice, nowhere in core code
 performs backward scans after having used heap_setscanlimits(), yet, it is
 possible an extension uses the heap functions in this way, hence
 backpatch.
 
 An upcoming patch does use heap_setscanlimits() with backward scans, so
 this must be fixed before that can go in.
 
 Author: David Rowley
 Discussion: https://postgr.es/m/CAApHDvpGc9h0_oVD2CtgBcxCS1N-qDYZSeBRnUh+0CWJA9cMaA@mail.gmail.com
 Backpatch-through: 9.5, all supported versions