libdvdread: src/ifo_read.c Fix off-by-one error on displaying IFO filename

Fixes incorrect warning:

libdvdread: Invalid IFO for title 2 (VTS_02_0.BU).

to:

libdvdread: Invalid IFO for title 2 (VTS_02_0.BUP).

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>