rd_pick_palette_intra_sby() was being passed a dummy 'best_mbmi' in
one case, and later that dummy value could be assigned to 'mbmi'
causing invalid values in 'mbmi'.

This bug was introduced by:
de0c70a2: Refactor rd_pick_intra_sby_mode()

See the referenced bug for a repro case.

BUG=aomedia:337

Change-Id: If6b0aa329f4e4902cd4de8a5e09bc5d9196492a1