http_negotiate: do not treat failure of gss_init_sec_context() as fatal

Fixes #3726
Closes #3849

lib/http_negotiate.c

@@ -143,7 +143,13 @@ CURLcode Curl_output_negotiate(struct connectdata *conn, bool proxy)
     }
     if(!neg_ctx->context) {
       result = Curl_input_negotiate(conn, proxy, "Negotiate");
-      if(result)
+      if(result == CURLE_LOGIN_DENIED) {
+        /* negotiate auth failed, let's continue unauthenticated to stay
+         * compatible with the behavior before curl-7_64_0-158-g6c6035532 */
+        conn->data->state.authproblem = TRUE;
+        return CURLE_OK;
+      }
+      else if(result)
         return result;
     }