Fix DROP INDEX CONCURRENTLY IF EXISTS.

This threw ERROR, not the expected NOTICE, if the index didn't exist.
The bug was actually visible in not-as-expected regression test output,
so somebody wasn't paying too close attention in commit
8cb53654db.
Per report from Brendan Byrd.

src/backend/parser/gram.y

@@ -4941,7 +4941,7 @@ DropStmt:	DROP drop_type IF_P EXISTS any_name_list opt_drop_behavior
 				{
 					DropStmt *n = makeNode(DropStmt);
 					n->removeType = OBJECT_INDEX;
-					n->missing_ok = FALSE;
+					n->missing_ok = TRUE;
 					n->objects = $6;
 					n->arguments = NIL;
 					n->behavior = $7;