blocks/tagged_stream_multiply_length: Fix tag duplication issues

ninput_items[0] can be larger than noutput_items and so tags between
[nitems_read(0)+n_output_items -> nitems_read(0)+ninput_items(0) ) will
be added once in that iteration and then again in the next ones (possibly
several times over)

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>