lavfi/aevalsrc: try to honor specified duration

If nb_samples divide sample_rate and if nb_samples allow it, aevalsrc
should generate the exact amount of samples according to duration.

Example:
aevalsrc=0::n=480:s=48000:d=5.21 should generate 250080 samples.

Signed-off-by: Stefano Sabatini <stefasab@gmail.com>