Fix have_topright check in check_intra_mode().

Prevents reading from uninitialized pointer 'top'.
Don't backup intra pre-lf intra pred borders for last SB row.

The frame height might not be a multiple of 64, and so this data might
be outside the allocated buffers of the image. Since this is the last
row of the image, we therefore skip backing up of the pre-lf intra pred
data.