Fix a bug for multi_arf_allowed

Fix a bug for multi_arf_allowed

The ARF Index was wrong when updating the upsampled reference
frame buffer.

Compared to the baseline in which multi_arf_allowed is disabled, the
RD performance drops 2.250% in Avg using Overall PSNR in the derf
dataset. The performance decrease is especially in the following
video sequences:

foreman_cif: drops 7.489%
husky_cif: drops 6.421%
soccer_cif: drops 4.850%

However, it has a significant gain in the following video sequences:

container_cif: increases 8.043%
harbour_cif: increases 1.332%

Change-Id: I02472909eb34bd070d7544f57383e72559fa42b3