Fix GiST index deletion assert issue.

Avoid calling heap_index_delete_tuples() with an empty deltids array to
avoid an assertion failure.

This issue was arguably an oversight in commit b5f58cf2, though the
failing assert itself was added by my recent commit d168b666.  No
backpatch, though, since the oversight is harmless in the back branches.

Author: Peter Geoghegan <pg@bowt.ie>
Reported-By: Jaime Casanova <jcasanov@systemguards.com.ec>
Discussion: https://postgr.es/m/CAJKUy5jscES84n3puE=sYngyF+zpb4wv8UMtuLnLPv5z=6yyNw@mail.gmail.com

src/backend/access/index/genam.c

@@ -301,6 +301,8 @@ index_compute_xid_horizon_for_tuples(Relation irel,
 	Page		ipage = BufferGetPage(ibuf);
 	IndexTuple	itup;
 
+	Assert(nitems > 0);
+
 	delstate.bottomup = false;
 	delstate.bottomupfreespace = 0;
 	delstate.ndeltids = 0;