rt_linux: fix buffer_frames overflow panic

[hinto-janai]

promote_current_thread_to_real_time() will panic if:

• The given audio_buffer_frames is 0 and audio_samplerate_hz is greater than 85_880 OR
• The given audio_buffer_frames is 4295 or greater

Line 274 is where the multiplication overflow panic occurs.

audio_thread_priority/src/rt_linux.rs

Lines 268 to 274 in 0399abb

	let buffer_frames = if audio_buffer_frames > 0 {
	audio_buffer_frames
	} else {
	// 50ms slice. This "ought to be enough for anybody".
	audio_samplerate_hz / 20
	};
	let budget_us = (buffer_frames * 1_000_000 / audio_samplerate_hz) as u64;

For example, using a relatively normal input 0 (default) and 96,000Hz:

// This panics.
//
// This will eventually execute:
//   96_000 / 20 = 4800
//   4800 * 1_000_000 = 4_800_000_000
//
// which overflows `u32`.
audio_thread_priority::promote_current_thread_to_real_time(0, 96_000);

[kinetiknz]

Thanks for this! Merged and added a simple test to avoid repeating this bug.