With ext interp, write_switchable_interp_filter calls
vp10_is_interp_needed, which needs access to the reference frame
buffers to check if they are scaled, the ref frame buffer pointer
at this point used to be uninitialized in the encoder resulting in
bitstream syntax mismatch when the encoder/decoder did not read/write
the interp filter element consistently.

Change-Id: Ie0be2a19cbfcb5639a751aa857458e91c23b8fe3