Marking an object as dependant on an extension did not have any
privilege check whatsoever; this allowed any user to mark objects as
droppable by anyone able to DROP EXTENSION, which could be used to cause
system-wide havoc.  Disallow by checking that the calling user owns the
mentioned object.

(No constraints are placed on the extension.)

Security: CVE-2020-1720
Reported-by: Tom Lane
Discussion: 31605.1566429043@sss.pgh.pa.us