BUG #18888: /src/interfaces/ecpg/preproc/descriptor.c possible NULL dereference

The following bug has been logged on the website:

Bug reference:      18888
Logged by:          Daniel Elishakov
Email address:      dan-eli@mail.ru
PostgreSQL version: 17.4
Operating system:   Ubuntu 20.04
Description:        

On 203 and 313 lines It seems that a struct data type should not be used in
'EXEC SQL SET DESCRIPTOR' command, so the code in question should never be
executed and it is not a problem. However there are no actual checks for
corrrectess of the provided data type. It is required to add a check against
wrong data types supplied by the user.