NAME_CONST(..) was used wrongly in a HAVING clause, and
should have caused a user error.  Instead, it caused a
segmentation fault.
      
During parsing, the value parameter to NAME_CONST was
specified to be an uninitialized Item_ref object (it
would be resolved later).   During the semantic analysis,
the object is tested, and since it was not initialied,
the server seg.faulted.
      
The fix is to check if the object is initialized
before testing it.  The same pattern has already been
applied to most other methods in the Item_ref class.
      
Bug was introduced by the optimization done as part of
Bug#33546.