libnm: assert in nm_utils_is_uuid() for valid argument

nm_utils_is_uuid() is public API. Commonly we check input arguments for such
functions with g_return_*() assertions.

libnm-core/nm-utils.c

@@ -4333,6 +4333,8 @@ nm_utils_is_uuid (const char *str)
 	const char *p = str;
 	int num_dashes = 0;
 
+	g_return_val_if_fail (str, FALSE);
+
 	while (*p) {
 		if (*p == '-')
 			num_dashes++;