Remove redundant 'version == NULL' check

*** CID 318094:  Null pointer dereferences  (REVERSE_INULL)
    /lib/dns/rbtdb.c: 1389 in newversion()
    1383     	version->xfrsize = rbtdb->current_version->xfrsize;
    1384     	RWUNLOCK(&rbtdb->current_version->rwlock, isc_rwlocktype_read);
    1385     	rbtdb->next_serial++;
    1386     	rbtdb->future_version = version;
    1387     	RBTDB_UNLOCK(&rbtdb->lock, isc_rwlocktype_write);
    1388
       CID 318094:  Null pointer dereferences  (REVERSE_INULL)
       Null-checking "version" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
    1389     	if (version == NULL) {
    1390     		return (result);
    1391     	}
    1392
    1393     	*versionp = version;
    1394

(cherry picked from commit 456d53d1)

Closes #2468 (closed)