httpd: refuse an unauthenticated firmware upload before it changes state

The multipart branch for /config tests authenticated before it touches
anything; the branch for /upload does not, and the 401 for it is only
raised further down at the dispatch. By then the request has already set
config_upload, uptr, verify_crc, max_upload and pre_acc, and announced a
firmware upload on the console.

Those five are globals while the transfer state they steer is per
connection, and the stack is built for ten connections, so a request that
is about to be rejected can still retarget an upload another client is in
the middle of: a configuration upload continues as a firmware one and its
remaining parts are written to the staging area instead of the
configuration sector.

Apply the same test the neighbouring branch uses, before the assignments.

httpd/httpd.c

@@ -750,6 +750,10 @@ void handle_post(void)
 			return;
 		}
 		if (is_word(request_path, "upload")) {
+			if (!authenticated) {
+				send_unauthorized();
+				return;
+			}
 			if (flash_size < FIRMWARE_UPLOAD_START*2)
 			{
 				print_string("Flash too small for firmware upload!\n");