mobile/gsm322.c: prevent buffer over-/under-run

Change-Id: Ic12587a6c6456b8663e5357cf68a22c6d1927a07

src/host/layer23/src/mobile/gsm322.c

@@ -320,6 +320,9 @@ static char *bargraph(int value, int min, int max)
 	else
 		value -= min;
 
+	/* Prevent 'bar' buffer over-/under-run */
+	OSMO_ASSERT(value >= 0 && value < 128);
+
 	/* Prevent calling memset() with zero length */
 	if (value == 0)
 		return "";