Extra } in Formatter::interpolate with numeric index

`\Phalcon\Logger\Formatter::interpolate` leaves a closing } behind when using numeric indices for `context`. That is, the following produces the message `Hello World}`:
```
<?php
$logger = new Phalcon\Logger\Adapter\Stream("php://stdout");
$logger->info("Hello {0}", ['World']);
```
This seems to happen because the return value of `spprintf` does not count the terminating NUL character.