check return value of stream_select

This change was already requested as part of https://github.com/reactphp/react/pull/297

In the current Implementation, the return value of stream_select is not checked at all. According to the [php documentation](http://php.net/manual/en/function.stream-select.php#refsect1-function.stream-select-returnvalues), a return value of `false` indicates that the underlying system call failed, e.g. due to a received signal. If thats the case, we cannot savely rely on the outcome of stream_select.

Unlike https://github.com/reactphp/react/pull/297, this PR does not supress any raised warning.