Fix undefined method error

[FixRM rapid7#8338]
[FixRM rapid7#8337]

Author: wchen-r7

modules/auxiliary/scanner/http/litespeed_source_disclosure.rb

@@ -62,7 +62,12 @@ def run_host(ip)
 				'uri'     => "#{uri}#{nullbytetxt}",
 			}, 25)
 
-			version = res.headers['Server'] if res
+			if res.nil?
+				print_error("Connection timed out")
+				return
+			end
+
+			version = res.headers['Server']
 
 			if vuln_versions.include?(version)
 				print_good("#{target_url} - LiteSpeed - Vulnerable version: #{version}")