Fix bytes_to_read adjustment in callback.
For example bug exposes itself with:
rtl_sdr -f 100e6 -s 2048000 -n 245.76e6 /tmp/100M.iq
and does not terminate after the expected 245.76e6 samples.
Which is an exact multiple of "out_block_size".
Signed-off-by: Henk vergonet <henk.vergonet(a)gmail.com>