[changeset] - have closereq() respect the property "tag"

[John W. Eaton]

On  9-Oct-2008, Ben Abbott wrote:

| Presently closereq() checks the handle value to be sure it is not  
| equal to 0.
| 
| Instead it should check that the handle is assigned to a figure.

Should we also avoid the error for (possibly) calling delete (0)?  I'm
not even sure that can happend, and at least calling delete (0)
doesn't actually delete the root figure, so I suppose this change is
OK.

Thanks,

jwe