MDEV-5668 Assertion `granted_role->is_role()' fails on granting role with empty name

parser: error out on empty role names

mysql-test/suite/roles/grant_empty.result

@@ -1,3 +1,5 @@
+grant '' to foo@localhost;
+ERROR OP000: Invalid role specification ``.
 create user ''@localhost;
 create role r1;
 grant r1 to ''@localhost;

mysql-test/suite/roles/grant_empty.test

@@ -1,3 +1,9 @@
+#
+# MDEV-5668 Assertion `granted_role->is_role()' fails on granting role with empty name
+#
+--error ER_INVALID_ROLE
+grant '' to foo@localhost;
+
 #
 # MDEV-5238 Server crashes in find_role_grant_pair on SHOW GRANTS for an anonymous user
 #

sql/sql_yacc.yy

@@ -15192,6 +15192,11 @@ current_role:
 grant_role:
           ident_or_text
           {
+            if ($1.length == 0)
+            {
+              my_error(ER_INVALID_ROLE, MYF(0), "");
+              MYSQL_YYABORT;
+            }
             if (!($$=(LEX_USER*) thd->alloc(sizeof(st_lex_user))))
               MYSQL_YYABORT;
             $$->user = $1;