admin console: log when JWTAuth::verify() doesn't have enough tokens

This is no longer a huge problem, but it's still a good idea to return
early in that case.

Found with the recently added admin_fuzzer, when I locally disabled the
StringVector safety checks for test purposes.

(If you view the diff with -U30, then you see that we access tokens[2]
later, so if size is < 3, we should give up.)

Change-Id: I46fc531fb042cc1485a17a9e994ad37e9ff0cd80
Reviewed-on: https://gerrit.libreoffice.org/c/online/+/91587
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

fuzzer/admin-data/crash-ba9e8295835983c8b0b3f92ec5ef65cc76e78f12

@@ -0,0 +1 @@
+auth jwt=eyJakilliwiZXhwIjoiMTU4NTU3O3hbGciOiJ€UzI1NiIsInR5MJ0.DM1NeyJpc3MiOiJiJ9

wsd/Auth.cpp

@@ -105,6 +105,12 @@ bool JWTAuth::verify(const std::string& accessToken)
 
     try
     {
+        if (tokens.size() < 3)
+        {
+            LOG_INF("JWTAuth: verification failed; Not enough tokens");
+            return false;
+        }
+
         const std::string encodedBody = tokens[0] + '.' + tokens[1];
         _digestEngine.update(encodedBody.c_str(), static_cast<unsigned>(encodedBody.length()));
         Poco::Crypto::DigestEngine::Digest digest = _digestEngine.signature();