Landing rapid7#1744 - Checks nil before using resp.headers['Server']

[Closes rapid7#1744]

Author: wchen-r7

modules/auxiliary/scanner/winrm/winrm_auth_methods.rb

@@ -37,7 +37,7 @@ def initialize
 	def run_host(ip)
 		resp = winrm_poke
 		return nil if resp.nil?
-		if  resp.code == 401 and resp.headers['Server'].include? "Microsoft-HTTPAPI"
+		if  resp.code == 401 and resp.headers['Server'] and resp.headers['Server'].include? "Microsoft-HTTPAPI"
 			methods = parse_auth_methods(resp)
 			desc = resp.headers['Server'] + " Authentication Methods: " + methods.to_s
 			report_service(