(dump): Correct loop-termination criterion.

(dump): Correct loop-termination criterion.
Before, running `printf 1234| ./od --width=4 --read-bytes=4'
printed output for 8 bytes -- the last four were garbage.
This happened only when the dump limit, N, was specified (with
--read-bytes=N) and N was a multiple of bytes_per_block
(usually 16, but 4 in this example).  From Andreas Schwab.