Window functions: return error if aggregate is not supported as window functions

mysql-test/r/win.result

@@ -1831,3 +1831,13 @@ rank() over (order by b)
 0
 0
 drop table t1;
+#
+# MDEV-9894: Assertion `0' failed in Window_func_runner::setup
+#  return ER_NOT_SUPPORTED_YET for aggregates that are not yet supported
+#  as window functions.
+#
+create table t1 (i int);
+insert into t1 values (1),(2);
+SELECT MAX(i) OVER (PARTITION BY (i)) FROM t1;
+ERROR 42000: This version of MariaDB doesn't yet support 'This aggregate as window function'
+drop table t1;

mysql-test/t/win.test

@@ -1113,3 +1113,14 @@ from
 
 drop table t1;
 
+--echo #
+--echo # MDEV-9894: Assertion `0' failed in Window_func_runner::setup
+--echo #  return ER_NOT_SUPPORTED_YET for aggregates that are not yet supported
+--echo #  as window functions.
+--echo #
+create table t1 (i int);
+insert into t1 values (1),(2);
+--error ER_NOT_SUPPORTED_YET
+SELECT MAX(i) OVER (PARTITION BY (i)) FROM t1;
+drop table t1;
+

sql/sql_window.cc

@@ -1766,7 +1766,8 @@ bool Window_func_runner::setup(THD *thd)
       break;
     }
     default:
-      DBUG_ASSERT(0);
+      my_error(ER_NOT_SUPPORTED_YET, MYF(0), "This aggregate as window function"); 
+      return true;
   }
 
   return false;