Ensure that hex strings are used as integers in cast(... signed/unsigned)

This fixes the new report for bug #7036

mysql-test/t/cast.test

@@ -20,6 +20,10 @@ select CONVERT(DATE "2004-01-22 21:45:33",CHAR);
 select CONVERT(DATE "2004-01-22 21:45:33",CHAR(4));
 select CONVERT(DATE "2004-01-22 21:45:33",BINARY(4));
 select CAST(DATE "2004-01-22 21:45:33" AS BINARY(4));
+select CAST(0xb3 as signed);
+select CAST(0x8fffffffffffffff as signed);
+select CAST(0xffffffffffffffff as unsigned);
+select CAST(0xfffffffffffffffe as signed);
 
 # out-of-range cases
 select cast('18446744073709551616' as unsigned);

sql/item.h

@@ -842,6 +842,7 @@ class Item_varbinary :public Item
   String *val_str(String*) { DBUG_ASSERT(fixed == 1); return &str_value; }
   int save_in_field(Field *field, bool no_conversions);
   enum Item_result result_type () const { return STRING_RESULT; }
+  enum Item_result cast_to_int_type() const { return INT_RESULT; }
   enum_field_types field_type() const { return MYSQL_TYPE_STRING; }
   // to prevent drop fixed flag (no need parent cleanup call)
   void cleanup() {}