Fix parsing of DROP SUBSCRIPTION ... DROP SLOT

It didn't actually parse before.
Reported-by: Masahiko Sawada <sawada.mshk@gmail.com>

src/backend/parser/gram.y

@@ -9196,11 +9196,13 @@ DropSubscriptionStmt: DROP SUBSCRIPTION name opt_drop_slot
 		;
 
 opt_drop_slot:
-			IDENT SLOT
+			DROP SLOT
 				{
-					if (strcmp($1, "drop") == 0)
-						$$ = TRUE;
-					else if (strcmp($1, "nodrop") == 0)
+					$$ = TRUE;
+				}
+			| IDENT SLOT
+				{
+					if (strcmp($1, "nodrop") == 0)
 						$$ = FALSE;
 					else
 						ereport(ERROR,

src/test/subscription/t/001_rep_changes.pl

@@ -179,7 +179,7 @@ $node_publisher->poll_query_until('postgres',
   or die "Timed out while waiting for apply to restart";
 
 # check all the cleanup
-$node_subscriber->safe_psql('postgres', "DROP SUBSCRIPTION tap_sub_renamed");
+$node_subscriber->safe_psql('postgres', "DROP SUBSCRIPTION tap_sub_renamed DROP SLOT");
 
 $result =
   $node_subscriber->safe_psql('postgres', "SELECT count(*) FROM pg_subscription");