wayland: Improve EOF detection when reading selections
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.
Closes: #1 (closed)