mobile/gsm322.c: prevent calling memset() with zero length

This change prevents a possibility of calling memset()
with constant zero length parameter, and the corresponding
compiler warning.

Change-Id: I2d8d78474614939659a7f24d5007b1c890776b1a

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

@@ -320,6 +320,10 @@ static char *bargraph(int value, int min, int max)
 	else
 		value -= min;
 
+	/* Prevent calling memset() with zero length */
+	if (value == 0)
+		return "";
+
 	memset(bar, '=', value);
 	bar[value] = '\0';