Fix memory leak for vp8 multi-res encoder.
Fix memory leak for vp8 multi-res encoder.

BUG=webm:1630

Change-Id: I03e74e78aa0ead66eda7506e921b1774b5442ed5

webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx