Fix rapid7#4089 - undefined method `downcase' for nil:NilClass

Author: wchen-r7

lib/rex/proto/smb/simpleclient.rb

@@ -66,6 +66,10 @@ def login(name = '', user = '', pass = '', domain = '',
 
       self.client.spnopt = spnopt
 
+      # In case the user unsets the password option, we make sure this is
+      # always a string
+      pass ||= ''
+
       ok = self.client.session_setup(user, pass, domain)
     rescue ::Interrupt
       raise $!