Avoid unnecessary MemSet call

The variable in question was changed from a struct to a pointer some
time ago (77947c51c0).  Using MemSet to zero it still works but is
obviously unidiomatic and confusing, so change it to a straight
assignment.

Author: Ranier Vilela <ranier.vf@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAEudQApCeq4JjW1BdnwU=m=-DvG5WyUik0Yfn3p6UNphiHjj+w@mail.gmail.com

src/backend/utils/cache/relcache.c

@@ -6258,7 +6258,7 @@ load_relcache_init_file(bool shared)
 		rel->rd_firstRelfilenodeSubid = InvalidSubTransactionId;
 		rel->rd_droppedSubid = InvalidSubTransactionId;
 		rel->rd_amcache = NULL;
-		MemSet(&rel->pgstat_info, 0, sizeof(rel->pgstat_info));
+		rel->pgstat_info = NULL;
 
 		/*
 		 * Recompute lock and physical addressing info.  This is needed in