[utilities][ulog] fix ulog backend color log length calculation error

Signed-off-by: chenyong <chenyong@rt-thread.com>

components/utilities/ulog/ulog.c

@@ -408,7 +408,11 @@ void ulog_output_to_all_backend(rt_uint32_t level, const char *tag, rt_bool_t is
         else
         {
             /* recalculate the log start address and log size when backend not supported color */
-            rt_size_t color_info_len = rt_strlen(color_output_info[level]), output_size = size;
+            rt_size_t color_info_len = 0, output_size = size;
+
+            if (color_output_info[level] != RT_NULL)
+                color_info_len = rt_strlen(color_output_info[level]);
+
             if (color_info_len)
             {
                 rt_size_t color_hdr_len = rt_strlen(CSI_START) + color_info_len;