Clean up computation of "from" and "to" values in AudioFileProcessorWaveView

Pull request #7071 has fixed a crash that was reported in #7068 by adding safety checks via two new setter methods in plugins/AudioFileProcessor/AudioFileProcessorWaveView.h.

While the crash is gone there is still code that tries to set the "from" and "to" values to invalid values. One such method is AudioFileProcessorWaveView::slide. To fix this adjust the two new setters as follows:

void AudioFileProcessorWaveView::setTo(f_cnt_t to)
{
	assert (to <= m_sample->sampleSize());
	m_to = std::min(to, static_cast<lmms::f_cnt_t>(m_sample->sampleSize()));
}

void AudioFileProcessorWaveView::setFrom(f_cnt_t from)
{
	assert (from >= 0);
	m_from = std::max(from, 0);
}

Then follow these steps:

1. Add an AFP and open it's view.
2. Drag a sample onto the sample view of the AFP, e.g. drums/clap01.ogg from the factory samples.
3. Click with the left mouse button on the sample display and drag to the right until the crash occurs.

Start in plugins/AudioFileProcessor/AudioFileProcessorWaveView.h and inspect the two setter methods, then trace how AudioFileProcessorWaveView::slide supplies its values. Reproduce the issue by opening an Audio File Processor, loading drums/clap01.ogg, and dragging across the sample display; done means invalid "from" and "to" values no longer lead to the reported crash.