Fix undefined method error

[FixRM rapid7#8349]

Author: wchen-r7

modules/auxiliary/admin/http/jboss_seam_exec.rb

@@ -90,7 +90,8 @@ def run
 					'method' => 'GET',
 				}, 20)
 
-			if (res.headers['Location'] =~ %r(pwned=java.lang.UNIXProcess))
+
+			if (res and res.headers['Location'] =~ %r(pwned=java.lang.UNIXProcess))
 				print_status("Exploited successfully")
 			else
 				print_status("Exploit failed.")