Misplaced superuser check in pg_log_backend_memory_contexts()

Hi all,

While reading the code of pg_log_backend_memory_contexts(), I have
been surprised to see that the code would attempt to look at a PROC
entry based on the given input PID *before* checking if the function
has been called by a superuser.  This does not strike me as a good
idea as this allows any users to call this function and to take
ProcArrayLock in shared mode, freely.

It seems to me that we had better check for a superuser at the
beginning of the function, like in the attached.

Thanks,
--
Michael