mixer: assert that the new volume value is valid

Added an assertion in mixer_set_volume().  Removed the range checks
from the ALSA and OSS plugins.

src/mixer/alsa_mixer.c

@@ -188,10 +188,6 @@ alsa_mixer_set_volume(struct mixer *mixer, unsigned volume)
 	vol = volume;
 
 	am->volume_set = vol + 0.5;
-	am->volume_set = am->volume_set > 100
-		? 100 :
-		(am->volume_set < 0
-		 ? 0 : am->volume_set);
 
 	level = (long)(((vol / 100.0) * (am->volume_max - am->volume_min) +
 			am->volume_min) + 0.5);

src/mixer/oss_mixer.c

@@ -171,9 +171,7 @@ oss_mixer_set_volume(struct mixer *mixer, unsigned volume)
 	int ret;
 
 	assert(om->device_fd >= 0);
-
-	if (volume > 100)
-		volume = 100;
+	assert(volume <= 100);
 
 	level = (volume << 8) + volume;
 

src/mixer_control.c

@@ -165,6 +165,7 @@ mixer_set_volume(struct mixer *mixer, unsigned volume)
 	bool success;
 
 	assert(mixer != NULL);
+	assert(volume <= 100);
 
 	if (mixer->plugin->global && !mixer->failed && !mixer_open(mixer))
 		return false;