Check nil for SRVHOST option

Author: wchen-r7

lib/msf/core/exploit/browser_autopwnv2.rb

@@ -659,7 +659,7 @@ def get_exploit_urls(cli, request)
           host = datastore['URIHOST']
         elsif cli
           host = cli.peerhost
-        elsif datastore['SRVHOST'] != '0.0.0.0'
+        elsif datastore['SRVHOST'] && datastore['SRVHOST'] != '0.0.0.0'
           host = datastore['SRVHOST']
         else
           host = Rex::Socket.source_address