ofSoundPlayer::setVolume(float) doesn't behave similarly on OS X and linux

Expected behavior: on OSX, when setVolume(x) is called with x negative, the volume is set to 0.

Bug: on Linux (ubuntu), negative arguments are discarded but the previous valid volume is kept.

A simple fix could be to constrain* the given volume:
https://github.com/openframeworks/openFrameworks/blob/master/libs/openFrameworks/sound/ofSoundPlayer.cpp#L118

Here is a suggestion:

void ofSoundPlayer::setVolume(float vol){
    if( player ){
        if (vol < 0) vol = 0;       // XXX suggestion
        if (vol > 1) vol = 1;       // XXX suggestion (optional)
        player->setVolume(vol);
    }
}

Let me know if a pull request with these suggestions would be useful.
Cedric.

*vol range is 0 to 1:
https://github.com/openframeworks/openFrameworks/blob/master/libs/openFrameworks/sound/ofSoundPlayer.h#L103

Start in libs/openFrameworks/sound/ofSoundPlayer.cpp at ofSoundPlayer::setVolume(float), then check the declared 0-to-1 range in libs/openFrameworks/sound/ofSoundPlayer.h. Compare the OS X and Linux behavior for negative and above-range values; done means invalid volumes are handled consistently without retaining a previous valid volume.