tiny_ssim: Fix an 'Uninitialized argument value' bug
tiny_ssim: Fix an 'Uninitialized argument value' bug

found by clang-7.0.1 static analysis.
BUG=webm:1616

Change-Id: I2f7d1376e82e35227ad96d34417014ce5680ad96

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