Fix memory leak in get_peak_sample()

[Taka-Kazu]

This PR fixes a memory leak in get_peak_sample().

My environment

• Alpine linux 3.14 (using Docker)
  • python 3.9.5
  • pulseaudio 14.2

How to reproduce

Run below script and check the memory usage of the process. With master, the memory usage will continuously increase.

#!/usr/bin/env/python

import pulsectl

if __name__ == '__main__':
    with pulsectl.Pulse('test-pulse') as pulse:
        while True:
            pulse.get_peak_sample(source=None, timeout=0.1)

[mk-fg]

Thank you!