Fix #2085 About director segfault in cram-md5 function

Fix #2085 About director segfault in cram-md5 function

The socket is not supposed to be NULL at that point.

  BSOCK::recv (this=0x0) at bsock.c:795
  in cram_md5_respond (bs=0x0, password=0x7f0961a939c8 "336e56dd67724e551c983a1c50c1f51f", tls_remote_need=<value optimized out>, compatible=0x7f09a53e0a68) at cram-md5.c:155
  in authenticate_storage_daemon (jcr=0x1469218, store=0x7f0961a93a28) at authenticate.c:98
  in connect_to_storage_daemon (jcr=0x1469218, retry_interval=10, max_retry_time=1800, verbose=1) at msgchan.c:133
  in do_backup (jcr=0x1469218) at backup.c:498