Assert name/short_desc to prevent SHOW ALL segfault

The DefineCustomStringVariable function(or any other DefineCustomXXXVariable) has a short_desc parameter that can be NULL and it's not apparent that this will lead to a segfault when SHOW ALL is used.

This happens because the ShowAllGUCConfig function expects a non-NULL short_desc.

This happened for the Supabase supautils extension(https://github.com/supabase/supautils/issues/24) and any other extension that uses the DefineCustomXXXVariable has the same bug risk.

This patch does an Assert on the short_desc(also on the name as an extra measure), so a postgres built with --enable-cassert can prevent the above issue.

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Steve Chavez

Engineering at https://supabase.com/