Fix off-by-one in pg_xlogdump's fuzzy_open_file().

Fix off-by-one in pg_xlogdump's fuzzy_open_file().

In the unlikely case of stdin (fd 0) being closed, the off-by-one
would lead to pg_xlogdump failing to open files.

Spotted by Coverity.

Backpatch to 9.3 where pg_xlogdump was introduced.