Fix indefinite hangs when reading form closed files on some platforms

Check for end of file before awaiting

On Linux, awaiting for read on an EOF returns immediately; on Mac, it
never returns. This is a regression caused by the recent fix to not
specify invalid flags to `stream_await()` - the invalid flags made it
/always/ return immediately, hiding this bug.

Test Plan:

`tests/filesystem/FileTest.php` freezes forever on mac without this
change; it passes with it.