get_value_string(): guard against NULL

get_value_string() conveniently prints the value number to a static buffer if
it is unknown in a value_string array. Do the same if the value_string array
pointer itself is NULL.

If a value string array is user supplied and might be NULL, one could add a
separate NULL check around it; but by making get_value_string() itself guard
against NULL, another static char buffer to print the value number is avoided.

Change-Id: Ie640e9258a959da8f4f9089478de993509853997
diff --git a/src/utils.c b/src/utils.c
index 0b2ed31..2d5bcb0 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -73,6 +73,9 @@
 {
 	int i;
 
+	if (!vs)
+		return NULL;
+
 	for (i = 0;; i++) {
 		if (vs[i].value == 0 && vs[i].str == NULL)
 			break;