gtkprintbackendfile: Fix infinite loop in _cairo_write()

gtkprintbackendfile: Fix infinite loop in _cairo_write()

It can happen if the io channel has been closed. In that case
g_io_channel_write_chars() returns early because of a g_return macro
that checks if the io channel is writable. When returning from g_return
macros, the bytes written output parameter is not updated and the error
is not filled, so the error is not detected and the written variable is
used uninitialized. We should check the return value of
g_io_channel_write_chars() to break the loop.

https://bugzilla.gnome.org/show_bug.cgi?id=685419