An after-fix for MDEV-12849 Out-of-range errors when casting hex-hybrid to SIGNED and UNSIGNED

1. Adding the forgotten "SET sql_mode=STRICT_ALL_TABLES" into the test.
2. STRICT_ALL_TABLES revealed that CAST(0xFFFFFFFF AS SIGNED),
   e.g. with a hex number with 8 hex digits, did not work well.
   Fixing Item_func_unsigned::create_tmp_field() and
   Item_func_unsigned::create_field_for_create_select() to handle
   this corner case.

mysql-test/t/cast.test

@@ -520,6 +520,7 @@ DROP TABLE t1;
 --echo # MDEV-12849 Out-of-range errors when casting hex-hybrid to SIGNED and UNSIGNED
 --echo #
 
+SET sql_mode=STRICT_ALL_TABLES;
 DELIMITER $$;
 CREATE PROCEDURE p1(hh TEXT)
 BEGIN
@@ -559,3 +560,4 @@ CALL p1('80FFFFFFFFFFFFFF');
 CALL p1('8FFFFFFFFFFFFFFF');
 
 DROP PROCEDURE p1;
+SET sql_mode=DEFAULT;

sql/item_func.h

@@ -834,6 +834,14 @@ class Item_func_signed :public Item_int_func
     unsigned_flag= 0;
   }
   const char *func_name() const { return "cast_as_signed"; }
+  Field *create_tmp_field(bool group, TABLE *table)
+  {
+    return Item::create_tmp_field(false, table,
+                                  MY_INT32_NUM_DECIMAL_DIGITS - 2 +
+                                  unsigned_flag);
+  }
+  Field *create_field_for_create_select(TABLE *table)
+  { return Item_func_signed::create_tmp_field(false, table); }
   longlong val_int()
   {
     longlong value= args[0]->val_int_signed_typecast();