convolve_test: Fix high bit depth IOC runtime errors.
convolve_test: Fix high bit depth IOC runtime errors.

Add a cast.

BUG=webm:1225

Change-Id: I34ea18ee816569485c1f1046a81fd2a0ce527ac8

webmproject/libvpx,mbebenita/aom,smarter/aom,luctrudeau/aom,webmproject/libvpx,luctrudeau/aom,webmproject/libvpx,mbebenita/aom,mbebenita/aom,ShiftMediaProject/libvpx,luctrudeau/aom,mwgoldsmith/vpx,mwgoldsmith/libvpx,GrokImageCompression/aom,mbebenita/aom,mwgoldsmith/libvpx,Topopiccione/libvpx,smarter/aom,smarter/aom,mwgoldsmith/libvpx,mwgoldsmith/vpx,smarter/aom,Topopiccione/libvpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,ShiftMediaProject/libvpx,mbebenita/aom,mwgoldsmith/vpx,Topopiccione/libvpx,mbebenita/aom,ShiftMediaProject/libvpx,mbebenita/aom,mwgoldsmith/vpx,GrokImageCompression/aom,mwgoldsmith/libvpx,GrokImageCompression/aom,mwgoldsmith/vpx,GrokImageCompression/aom,Topopiccione/libvpx,mbebenita/aom,mbebenita/aom,ShiftMediaProject/libvpx,webmproject/libvpx,smarter/aom,GrokImageCompression/aom,smarter/aom,GrokImageCompression/aom,luctrudeau/aom,luctrudeau/aom,mwgoldsmith/libvpx,webmproject/libvpx,luctrudeau/aom,Topopiccione/libvpx,webmproject/libvpx,mwgoldsmith/libvpx