Fix smb capture error

The following error is observed in the auxiliary/server/capture/smb.rb when running Nessus with credentials against the SMB capture module:

[*] SMB Capture - Error processing Hash from 192.168.1.103:40389 : NoMethodError undefined method `+' for nil:NilClass ["/opt/metasploit/msf3/modules/auxiliary/server/capture/smb.rb:537:in `smb_get_hash'", "/opt/metasploit/msf3/modules/auxiliary/server/capture/smb.rb:401:in `smb_cmd_session_setup'", "/opt/metasploit/msf3/modules/auxiliary/server/capture/smb.rb:119:in `smb_cmd_dispatch'", "/opt/metasploit/msf3/lib/msf/core/exploit/smb.rb:748:in `smb_recv'", "/opt/metasploit/msf3/lib/msf/core/exploit/smb.rb:679:in `on_client_data'", "/opt/metasploit/msf3/lib/msf/core/exploit/tcp.rb:389:in `block in start_service'", "/opt/metasploit/msf3/lib/rex/io/stream_server.rb:48:in `call'", "/opt/metasploit/msf3/lib/rex/io/stream_server.rb:48:in `on_client_data'", "/opt/metasploit/msf3/lib/rex/io/stream_server.rb:192:in `block in monitor_clients'", "/opt/metasploit/msf3/lib/rex/io/stream_server.rb:190:in `each'", "/opt/metasploit/msf3/lib/rex/io/stream_server.rb:190:in `monitor_clients'", "/opt/metasploit/msf3/lib/rex/io/stream_server.rb:73:in `block in start'", "/opt/metasploit/msf3/lib/rex/thread_factory.rb:22:in `call'", "/opt/metasploit/msf3/lib/rex/thread_factory.rb:22:in `block in spawn'", "/opt/metasploit/msf3/lib/msf/core/thread_manager.rb:100:in `call'", "/opt/metasploit/msf3/lib/msf/core/thread_manager.rb:100:in `block in spawn'"]