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.

src/tail.c

@@ -818,7 +818,7 @@ start_bytes (const char *pretty_filename, int fd, uintmax_t n_bytes,
           error (0, errno, _("error reading %s"), quote (pretty_filename));
           return 1;
         }
-      read_pos += bytes_read;
+      *read_pos += bytes_read;
       if (bytes_read <= n_bytes)
         n_bytes -= bytes_read;
       else