Adding a fix for channel.fadeout using Mix_HaltChannel

[romgird2]

Hello,

In mixer_tests.py, we remarked that the tests test_fadeout_with_zero_time and test_fadeout_with_negative_time failed.

Investigating on the issue, we saw that the C function chan_fadeout in mixer.c does not check for <= 0 argument.

Thus, we propose to add a if statement in chan_fadeout to check for this condition and call Mix_HaltChannel in this case to stop the sound, as requested by the documentation.

Thanks for your consideration