swresample/swresample: Fix for seg fault in swr_convert_internal() -> sum2_float during dithering.

Removed +len1 in call to s->mix_2_1_f() as I found no logical explanation for it. After removal, problem was gone.

Signed-off-by: Hendrik Schreiber <hs@tagtraum.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>