MDEV-35677 Assertion `thd->is_error()' failed in virtual bool Field::sp_prepare_and_store_item(THD*, Item**)

The Count_handler (derived from Internal_error_handler) used in
Type_handler_datetime_common::convert_item_for_comparison()
suppressed all error codes. This caused a DBUG_ASSERT(thd->is_error())
in Field::sp_prepare_and_store_item() when the error happened
while evaluating a subselect in a scenario like this:

CREATE FUNCTION f() RETURNS INT DETERMINISTIC RETURN (SELECT a FROM t);
CREATE TABLE t (c TIMESTAMP);
SELECT * FROM t WHERE c=f();

Notice there is no such column 'a' in in the table 't'.

Fix:

Handle only DATETIME->TIMESTAMP conversion related errors in Count_handler:
- ER_TRUNCATED_WRONG_VALUE
- ER_DATETIME_FUNCTION_OVERFLOW
thus let other error kinds be processed in their usual way.

The reported scenario now returns this (expected) error:

ERROR 1054 (42S22): Unknown column 'a' in 'SELECT'

sql/sql_type.cc

@@ -7959,8 +7959,13 @@ Type_handler_datetime_common::convert_item_for_comparison(
                           const char *msg,
                           Sql_condition **cond_hdl) override
     {
-      hit++;
-      return *level >= Sql_condition::WARN_LEVEL_WARN;
+      if (sql_errno == ER_TRUNCATED_WRONG_VALUE ||
+          sql_errno == ER_DATETIME_FUNCTION_OVERFLOW)
+      {
+        hit++;
+        return *level >= Sql_condition::WARN_LEVEL_WARN;
+      }
+      return false;
     }
   } cnt_handler;