MDEV-26362: incorrect nest_level value with INTERSECT

Add DBUG_ASSERT (should be kept in merge)
Fix nest_level assignment in LEX::add_unit_in_brackets (should be ignored in merge to 10.4)

sql/sql_base.cc

@@ -7513,6 +7513,17 @@ bool setup_fields(THD *thd, Ref_ptr_array ref_pointer_array,
 
   thd->column_usage= column_usage;
   DBUG_PRINT("info", ("thd->column_usage: %d", thd->column_usage));
+  /*
+    Followimg 2 condition always should be true (but they was added
+    due to an error present only in 10.3):
+    1) nest_level shoud be 0 or positive;
+    2) nest level of all SELECTs on the same level shoud be equal first
+       SELECT on this level (and each other).
+  */
+  DBUG_ASSERT(thd->lex->current_select->nest_level >= 0);
+  DBUG_ASSERT(thd->lex->current_select->master_unit()->first_select()
+                ->nest_level ==
+              thd->lex->current_select->nest_level);
   if (allow_sum_func)
     thd->lex->allow_sum_func.set_bit(thd->lex->current_select->nest_level);
   thd->where= THD::DEFAULT_WHERE;

sql/sql_lex.cc

@@ -5288,7 +5288,6 @@ bool LEX::add_unit_in_brackets(SELECT_LEX *nselect)
   DBUG_ASSERT(nselect->outer_select() == dummy_select);
 
   current_select= dummy_select;
-  current_select->nest_level--;
 
   SELECT_LEX_UNIT *unit= nselect->master_unit();
   Table_ident *ti= new (thd->mem_root) Table_ident(unit);
@@ -5314,7 +5313,6 @@ bool LEX::add_unit_in_brackets(SELECT_LEX *nselect)
   derived_tables|= DERIVED_SUBQUERY;
 
   current_select= nselect;
-  current_select->nest_level++;
   DBUG_RETURN(rc);
 }