vp8 rdopt.c: zero rd.[rate_uv|distortion_uv]

vp8 rdopt.c: zero rd.[rate_uv|distortion_uv]

These values are not consistently set before calling update_best_mode.

In vp9_rdopt.c they are individual values instead of a struct and are
zero'd at declaration.

Clears a static analysis warning:
warning: The right operand of '-' is a garbage value
RDCOST(x->rdmult, x->rddiv, (rd->rate2 - rd->rate_uv - other_cost),

warning: The right operand of '-' is a garbage value
(rd->distortion2 - rd->distortion_uv));

Change-Id: I19895d062e7c0ac67937126ebc5dcb0afd3a2931