Change some unnecessary MemSet calls

Change some unnecessary MemSet calls

MemSet() with a value other than 0 just falls back to memset(), so the
indirection is unnecessary if the value is constant and not 0.  Since
there is some interest in getting rid of MemSet(), this gets some easy
cases out of the way.  (There are a few MemSet() calls that I didn't
change to maintain the consistency with their surrounding code.)

Discussion: https://www.postgresql.org/message-id/flat/CAEudQApCeq4JjW1BdnwU=m=-DvG5WyUik0Yfn3p6UNphiHjj+w@mail.gmail.com