Check that nmap.registry.args exists before trying to access

nmap.registry.args["http.useragent"]. The args table doesn't exist
during --script-updatedb. The bug was found by Tom Sellers and fixed by
Jah.

CHANGELOG

@@ -7,7 +7,8 @@ o A new script argument, http.useragent, gives the ability to modify
   the User-Agent header sent by NSE from its default of "Mozilla/5.0
   (compatible; Nmap Scripting Engine; http://nmap.org/book/nse.html)".
   You can set it to the empty string to disable the User-Agent
-  entirely. [David]
+  entirely. A bug was found and fixed by Tom Sellers and Jah,
+  respectively. [David]
 
 o [Nmat] The --ssl, --output, and --hex-dump options now work with
   --exec and --sh-exec. This is done by forking a separate process to

nselib/http.lua

@@ -42,7 +42,7 @@ local have_ssl = (nmap.have_ssl() and pcall(require, "openssl"))
 
 local USER_AGENT
 do
-  local arg = nmap.registry.args["http.useragent"]
+  local arg = nmap.registry.args and nmap.registry.args["http.useragent"]
   if arg and arg == "" then
     USER_AGENT = nil
   elseif arg then