HTTP digest auth crash when auth header is missing

[nnposter]

NSE encounters an error if the code prescribes HTTP digest authentication but the target server does not supply header WWW-Authenticate:

/usr/bin/../share/nmap/nselib/sasl.lua:121: attempt to concatenate a nil value (field 'nonce')
stack traceback:
        /usr/bin/../share/nmap/nselib/sasl.lua:121: in method 'calcDigest'
        /usr/bin/../share/nmap/nselib/http.lua:1416: in function 'http.generic_request'
        /usr/bin/../share/nmap/nselib/http.lua:1766: in function 'http.get'
        (...tail calls...)

The following patch rectifies the issue:

--- a/nselib/http.lua
+++ b/nselib/http.lua
@@ -1404,7 +1404,7 @@
     options_with_auth_removed["auth"] = nil
     local r = generic_request(host, port, method, path, options_with_auth_remov
     local h = r.header['www-authenticate']
-    if not r.status or (h and not string.find(h:lower(), "digest.-realm")) then
+    if not (r.status and h and string.find(h:lower(), "digest.-realm")) then
       stdnse.debug1("http: the target doesn't support digest auth or there was 
       return http_error("The target doesn't support digest auth or there was an
     end

The patch will be applied after August 1, 2019, unless concerns are raised.