tests/lzma: Fix off-by-one in buffer size
tests/lzma: Fix off-by-one in buffer size

Coverity spotted that we had an off-by-one here since we were using
`i+1`.  Fix this by adding a `-1` to the bounds check.  Also use
`sizeof()` to ensure the data and size are coupled.

Coverity CID: 1452207

Closes: #1037
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree