[MERGE PG13] ysql: fix some compilation errors

Summary:
Fix two compilation errors:

- release build got -Wsometimes-uninitialized compilation error because of
  Assert(false) that is treated differently in release/debug builds
- gcc build got -Werror=comment because of bad comment

Test Plan:
Jenkins: compile only, rebase: pg13

    ./yb_build.sh fastdebug --gcc11 --no-initdb
    ./yb_build.sh release --clang15 --no-initdb

Reviewers: smishra, neil

Reviewed By: neil

Subscribers: yql

Differential Revision: https://phabricator.dev.yugabyte.com/D22577

src/postgres/src/backend/executor/ybc_fdw.c

@@ -645,7 +645,7 @@ ybcIterateForeignScan(ForeignScanState *node)
 }
 
 #ifdef NEIL_TODO
-/* Keep this code till I look at ybFetchNext()
+/* Keep this code till I look at ybFetchNext() */
 {
 	/* Clear tuple slot before starting */
 	slot = node->ss.ss_ScanTupleSlot;

src/postgres/src/backend/optimizer/plan/createplan.c

@@ -3520,10 +3520,12 @@ create_modifytable_plan(PlannerInfo *root, ModifyTablePath *best_path)
 	{
 		Assert(false);
 		/* YB_TODO(neil@yugabyte). Need rewrite - Code path wouldn't get here.
+		 * Still need to initialize for the compiler: -Wsometimes-uninitialized.
 		 *
 		 * subplan = (Plan *) make_result(result_tlist, NULL, NULL);
 		 * copy_generic_path_info(subplan, linitial(best_path->subpaths));
 		 */
+		subplan = NULL;
 	}
 	else
 	{