cifs: ensure that vol->username is not NULL before running strlen on it

Dan Carpenter says:

The patch 04febabc: "cifs: sanitize username handling" from Jan
17, 2012, leads to the following static checker warning:

	fs/cifs/connect.c:2231 match_session()
	error: we previously assumed 'vol->username' could be null (see line 2228)

fs/cifs/connect.c
  2219                  /* NULL username means anonymous session */
  2220                  if (ses->user_name == NULL) {
  2221                          if (!vol->nullauth)
  2222                                  return 0;
  2223                          break;
  2224                  }
  2225
  2226                  /* anything else takes username/password */
  2227                  if (strncmp(ses->user_name,
  2228                              vol->username ? vol->username : "",
                                    ^^^^^^^^^^^^^
We added this check for vol->username here.

  2229                              CIFS_MAX_USERNAME_LEN))
  2230                          return 0;
  2231                  if (strlen(vol->username) != 0 &&
                                   ^^^^^^^^^^^^^
But this dereference is not checked.

  2232                      ses->password != NULL &&
  2233                      strncmp(ses->password,
  2234                              vol->password ? vol->password : "",
  2235                              CIFS_MAX_PASSWORD_LEN))
  2236                          return 0;

...fix this by ensuring that vol->username is not NULL before running
strlen on it.

Signed-off-by: Jeff Layton <jlayton@poochiereds.net>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <smfrench@gmail.com>