string_utils: coding rules

Signed-off-by: 2xsec <dh48.jeong@samsung.com>

src/lxc/string_utils.c

@@ -774,7 +774,7 @@ char *must_concat(const char *first, ...)
 	}
 	va_end(args);
 
-	dest[cur_len] = 0;
+	dest[cur_len] = '\0';
 	return dest;
 }