At startup of the decoder, i_channels might be set to 0 if the decoder
hasn't processed the sent CODECCONFIG data yet. (In these cases, the
returned buffers are 0 bytes long so we don't miss anything either.)
A port reconfigure event is sent in these cases, so i_channels gets
initialized to the right value before we handle any later buffers with
actual content.
Signed-off-by: Martin Storsjö <martin@martin.st>