The name buffer_availabile was kinda confusing, so its been renamed
to buffer_data_size() to match buffer_data().
Also I added a comment to buffer_ensure_space because its behaviour
wasn't obvious.
When the converter fills the whole buffer without reading all input,
we need to enlarge the buffer. Otherwise we get an assertion failure
for `outbuf_size > 0' in g_converter_convert.
https://bugzilla.gnome.org/show_bug.cgi?id=619945
