Ensure we always have some target space in GConverterOutputStream

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
(cherry picked from commit 86592d6)

gio/gconverteroutputstream.c

@@ -418,6 +418,10 @@ g_converter_output_stream_write (GOutputStream *stream,
   converted_bytes = 0;
   while (!priv->finished && converted_bytes < to_convert_size)
     {
+      /* Ensure we have *some* target space */
+      if (buffer_tailspace (&priv->converted_buffer) == 0)
+	grow_buffer (&priv->converted_buffer);
+
       /* Try to convert to our buffer */
       my_error = NULL;
       res = g_converter_convert (priv->converter,
@@ -529,6 +533,10 @@ g_converter_output_stream_flush (GOutputStream  *stream,
   flushed = FALSE;
   while (!priv->finished && !flushed)
     {
+      /* Ensure we have *some* target space */
+      if (buffer_tailspace (&priv->converted_buffer) == 0)
+	grow_buffer (&priv->converted_buffer);
+
       /* Try to convert to our buffer */
       my_error = NULL;
       res = g_converter_convert (priv->converter,