tail: fix an inconsequential bug spotted by coverity
* src/tail.c (start_bytes): Increase *READ_POS (not READ_POS)
by the number of bytes read. This is a real bug that happens
to have no consequence in practice. First, this code is exercised
only when tailing-forever a non-regular file by bytes, and with a
start-relative offset, e.g., "mkfifo f; tail -f -c +3 f", but even
then, the invalid READ_POS value does not influence how tail works.
It is stored in the File_spec.size member, but that member is not
used at all in tail_forever_inotify, and in tail_forever, it is
used only when the File_spec refers to a regular file.