[finsh]Fixed a bug may cause stackover flow

add code:
if (line_buf == RT_NULL) return -RT_ENOMEM;

components/finsh/msh_file.c

@@ -101,6 +101,7 @@ int msh_exec_script(const char *cmd_line, int size)
         int length;
 
         line_buf = (char *) rt_malloc(RT_CONSOLEBUF_SIZE);
+        if (line_buf == RT_NULL) return -RT_ENOMEM;
 
         /* read line by line and then exec it */
         do