Fix double assignment.

Coverity complained about this, not without reason:

	OldMultiXactReader *state = state = pg_malloc(sizeof(*state));

(I'm surprised this is even legal C ... why is "state" in-scope
in its initialization expression?)

While at it, convert to use our newly-preferred "pg_malloc_object"
macro instead of an explicit sizeof().

src/bin/pg_upgrade/multixact_read_v18.c

@@ -119,7 +119,7 @@ OldMultiXactReader *
 AllocOldMultiXactRead(char *pgdata, MultiXactId nextMulti,
 					  MultiXactOffset32 nextOffset)
 {
-	OldMultiXactReader *state = state = pg_malloc(sizeof(*state));
+	OldMultiXactReader *state = pg_malloc_object(OldMultiXactReader);
 	char		dir[MAXPGPATH] = {0};
 
 	state->nextMXact = nextMulti;