alsamixer: fix calculation in set_normalized_volume (overflow)

Lowering volume below 0 causes overflow spike to 100% volume (volume goes
below 0 and back to 100 repeatedly). 0 overflows past infinity when holding
down z,x,c.

> value = lrint_dir(6000.0 * log10(volume), dir) + max;
   (where volume = 0 , and dir = -1 . min = -9999999 , and max = -6)
> log10(0) is negative infinity = error

Fixes: #266
Reported-by: genr8eofl <genBTC@gmx.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>

alsamixer/volume_mapping.c

@@ -146,6 +146,8 @@ static int set_normalized_volume(snd_mixer_elem_t *elem,
 		min_norm = pow(10, (min - max) / 6000.0);
 		volume = volume * (1 - min_norm) + min_norm;
 	}
+	if (volume <= 0)
+		volume = 1e-36;
 	value = lrint_dir(6000.0 * log10(volume), dir) + max;
 	return set_dB[ctl_dir](elem, channel, value, dir);
 }