ext/ftp: declare the false returned by ftp_nb_fget() and ftp_nb_fput()

[lacatoire]

ftp_nb_fget() and ftp_nb_fput() are declared int, but both answer an already busy connection with false:

if (ftp->in_use) {
	php_error_docref(NULL, E_WARNING, "FTP\\Connection is already in use");
	RETURN_FALSE;
}

ftp_nb_get() and ftp_nb_put() carry the same guard and already declare int|false; the two stream variants were left behind. Internal functions do not coerce their return value, so a caller hitting the guard receives false against a declaration promising int, and === FTP_FAILED (integer 0) does not match it.

ftp_nb_continue() has no such guard — every exit is a RETURN_LONG — so it stays int.

Only the declarations change. in_use is set and cleared within a single engine call, so the guard is reachable through re-entrancy alone and nothing at runtime moves. The added test reads the declared types of all five ftp_nb_* functions and needs no FTP server.

[ndossche]

Perhaps the code should be revised to throw an Error rather than returning false. Generally, Error is used to indicate a programming mistake, and this is the case here. I believe that's more appropriate than widening the return type

[ndossche]

cc @iliaal