Use pg_memory_is_all_zeros() in pgstatfuncs.c.

Use pg_memory_is_all_zeros() in pgstatfuncs.c.

There are a few places in this file that use memset() and memcmp()
to determine whether a section of memory is all zeros.  This commit
modifies them to use pg_memory_is_all_zeros() instead.  These
aren't expected to be hot code paths, but this may optimize them a
bit.  Plus, this allows us to remove some variables that were only
needed for the memset() and memcmp().

Author: Bertrand Drouvot
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/Z1hNubHfvMxlW6eu%40ip-10-97-1-34.eu-west-3.compute.internal