fix possible NULL dereference in `cfg_map_findclause()`

fix possible NULL dereference in `cfg_map_findclause()`

`cfg_map_findclause()` did not check whether a clause existed before
dereferencing it, which could lead to a NULL dereference. Add the
missing check to prevent this.

In practice, this was not triggering any known bug, since
`cfg_map_findclause()` is only called in contexts where the clause is
known to exist.