The problem is that if a NULL is stored in an Item_cache_decimal object,
the associated my_decimal object is not initialized.  However, it is still
accessed when val_int() is called. The fix is to check for null_value
within val_int(), and return without accessing the my_decimal object when
the cached value is NULL.

Bug#52122 reports the same issue for val_real(), and this patch also includes
fixes for val_real() and val_str() and corresponding test cases from that
bug report.  

Also, NULL is returned from val_decimal() when value is null. This will
avoid that callers access an uninitialized my_decimal object.

Made similar changes to all other Item_cache classes.  Now all val_*
methods should return a well defined value when actual value is NULL.

mysql-test/r/type_decimal.result:
  Updated result file with test cases for Bug#52168 and Bug#52122.
mysql-test/t/type_decimal.test:
  Added test cases for Bug#52168 and Bug#52122.
sql/item.cc:
  In Item_cache_*::val_* methods, return a well defined value
  when actual value is NULL.
  
  This is especially important for Item_cache_decimal since
  otherwise one risk accessing an uninitialized my_decimal object.
sql/item.h:
  Added method Item_cache::has_value() which returns TRUE if cache 
  object contains a non-null value.