Avoid a crash by making sure response *has* a location header

scripts/https-redirect.nse

@@ -58,6 +58,7 @@ action = function (host, port)
     if string.match(result, "^HTTP/1.[01] 3%d%d") then
 
       local location = string.match(result, "\n[Ll][Oo][Cc][Aa][Tt][Ii][Oo][Nn]:[ \t]*(.-)\r?\n")
+      if location then
         local parsed = url.parse(location)
         -- Check for a redirect to the same port, but with HTTPS scheme.
         if parsed.scheme == 'https' and tonumber(parsed.port or 443) == port.number and (
@@ -73,4 +74,5 @@ action = function (host, port)
         end
       end
     end
+  end
 end