g_input_stream_read_bytes() roughly provides the same guarantees
than g_input_stream_read() wrt the number of bytes being possibly
read (i.e. it being a best effort, but no real guarantees).

Instead, rely on the 0-len read that we'd get at the end of the
transfer.

Fixes clipboard/DnD transfers possibly being cut short, resulting
on "Broken pipe" errors on the other side.

GNOME/gtk#1

Closes: #1