dr_prediction_z2: check that index into 'left' is valid.

For 'left' array indices -1 onwards are valid (left[-1] == above[-1] ==
the top-left reference pixel).
So, make sure that 'base2' is in that range too.

Change-Id: If5118d97d4ef85f6520363bd648bfe6948c1211f
1 file changed