cifs: remove redundant check for null string pointer

server_RFC1001_name is declared as a RFC1001_NAME_LEN_WITH_NULL sized
char array in struct TCP_Server_Info so the null pointer check on
server_RFC1001_name is redundant and can be removed.  Detected with
smatch:

fs/cifs/connect.c:2982 ip_rfc1001_connect() warn: this array is probably
  non-NULL. 'server->server_RFC1001_name'

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Steve French <smfrench@gmail.com>