Fix losing data when reading from STDIN.

[sgebbie]

The current implementation fills the full underlying array based on the
"space" available.

However, the space may be larger than the chunk size since space is
allocated in powers of two. However, when the data buffer is passed back
via the InputNotify handle it is first appropriately truncated to the
actual len read by @pony_os_stdin_read. But, the implementation of
truncate sets the size to max(len, size) and does not take into account
the underlying allocated space.

The net result is that reading of stdin looses data unless the chunk
size provided is actually a power of two — in which case
_size == _alloc.

This change simply limits the read to data.size() rather than
data.space().

Please see: #2871.

[mfelsche]

Thank you! Good catch!
That makes very much sense.

Nonetheless I do have a bad feeling that we don't have no tests for this runtime behaviour.

We should at some point include running compiled pony programs and assert on their behaviour as a kind of integration test. But this is a story for another time.