Remove redundant listener != NULL check

With isc_mem_get no longer failing this check is no longer needed.

1258        } else {
    	
CID 304936 (#1 of 1): Dereference before null check (REVERSE_INULL)
check_after_deref: Null-checking listener suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
1259                if (listener != NULL) {
1260                        listener->exiting = true;
1261                        free_listener(listener);
1262                }