nshlib:judge whether nsh login username is a valid input

Signed-off-by: huangjian <huangjian@xiaomi.com>

nshlib/nsh_login.c

@@ -178,6 +178,12 @@ int nsh_login(FAR struct console_stdio_s *pstate)
           nsh_token(pstate, username, sizeof(username));
         }
 
+      if (username[0] == '\0')
+        {
+          i--;
+          continue;
+        }
+
 #ifdef CONFIG_NSH_PLATFORM_CHALLENGE
       platform_challenge(challenge, sizeof(challenge));
       fputs(challenge, pstate->cn_outstream);