libext2fs: avoid potential out-of-bounds write if pread/pread64 fails

libext2fs: avoid potential out-of-bounds write if pread/pread64 fails

In unix_io.c's raw_read_block(), if the initial attempt to call
pread/pread64 fails because the offset is insane, the variable
"actual" is left at -1, and then when lseek fails, the cleanup
function will try to clear (as an out-of-bounds write) a single byte
before the buffer.  Fix this.

Addresses-Debian-Bug: #871539

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reported-by: Jakub Wilk <jwilk@jwilk.net>