Check get_tle_by_resno() result before deref

When creating a sort to support a group by, we need to look up the
target entry in the target list by the resno using get_tle_by_resno().
This particular code-path didn't check the result prior to attempting
to dereference it, while all other callers did.  While I can't see a
way for this usage of get_tle_by_resno() to fail (you can't ask for
a column to be sorted on which isn't included in the group by), it's
probably best to check that we didn't end up with a NULL somehow
anyway than risk the segfault.

I'm willing to back-patch this if others feel it's necessary, but my
guess is new features are what might tickle this rather than anything
existing.

Missing check spotted by the Coverity scanner.

src/backend/optimizer/plan/createplan.c

@@ -4188,6 +4188,9 @@ make_sort_from_groupcols(PlannerInfo *root,
 		SortGroupClause *grpcl = (SortGroupClause *) lfirst(l);
 		TargetEntry *tle = get_tle_by_resno(sub_tlist, grpColIdx[numsortkeys]);
 
+		if (!tle)
+			elog(ERROR, "could not retrive tle for sort-from-groupcols");
+
 		sortColIdx[numsortkeys] = tle->resno;
 		sortOperators[numsortkeys] = grpcl->sortop;
 		collations[numsortkeys] = exprCollation((Node *) tle->expr);