Fix assertion

[YichiZhang0613]

In rodio/src/source/signal_generator.rs, frequency should be > 0 according to semantic but code allows it to be negative, so assertion should be changed into assert!(frequency > 0.0, "frequency must be greater than zero");

pub fn with_function(
    sample_rate: SampleRate,
    frequency: f32,
    generator_function: GeneratorFunction,
) -> Self {
    assert!(frequency != 0.0, "frequency must be greater than zero");
    let period = sample_rate as f32 / frequency;

[yara-blue]

thanks! 👍