lavfi/apad: fix logic when whole_len or pad_len options are specified

In particular, allow pad_len and whole_len to have value set to 0, which
means that no padding will be added. Previously a value set to 0 meant
that that the filter had to pad forever.

The new semantics is clearer, also simplifies scripting since the option
value might be automatically computed, so that no checks need to be done
in case it is 0.

The old semantics was never documented and the logic was broken (the
filter was always adding samples indefinitely), so this should not break
backward compatibility.
