util/fifo8: Allow fifo8_pop_buf() to not populate popped length

util/fifo8: Allow fifo8_pop_buf() to not populate popped length

There might be cases where we know the number of bytes we can
pop from the FIFO, or we simply don't care how many bytes is
returned. Allow fifo8_pop_buf() to take a NULL numptr.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231109192814.95977-2-philmd@linaro.org>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>