Use creds username as SNMP community if no ':' indicates password. Fixes #862

CHANGELOG

@@ -1,5 +1,9 @@
 # Nmap Changelog ($Id$); -*-text-*-
 
+o [NSE][GH#862] SNMP scripts will now take a community string provided like
+  `--script-args creds.snmp=private`, which previously did not work because it
+  was interpreted as a username. [Daniel Miller]
+
 o [NSE] Resolved several issues in the default HTTP redirect rules:
     - [GH#826] A redirect is now cancelled if the original URL contains
       embedded credentials

nselib/snmp.lua

@@ -447,9 +447,14 @@ Helper = {
       local creds_store = creds.Credentials:new(creds.ALL_DATA, host, port)
       for _,cs in ipairs({creds.State.PARAM, creds.State.VALID}) do
         local account = creds_store:getCredentials(cs)()
-        if (account and account.pass) then
-          o.community = account.pass == "<empty>" and "" or account.pass
+        if account then
+          if account.pass and account.pass ~= "<empty>" and account.pass ~= "" then
+            o.community = account.pass
             break
+          elseif account.user then
+            o.community = account.user
+            break
+          end
         end
       end
     end