Fix encoder uninitialized read errors reported by drmemory
Fix encoder uninitialized read errors reported by drmemory

This patch fixed the uninitialized read errors in Issue 748:
"dr memory VP9 encode errors". In vp9_convolve_avg_sse2,
when width is 4, pavgb reads 8 bytes from dst buffer that is
out of range. An error is reported although the data is not
actually used later. This issue was resolved by preventing
uninitialized reads.

Change-Id: I109a54910aa47139cb13119de86f2062cff207df

GrokImageCompression/aom,abwiz0086/webm.libvpx,zofuthan/libvpx,Topopiccione/libvpx,ittiamvpx/libvpx-1,felipebetancur/libvpx,shacklettbp/aom,Distrotech/libvpx,GrokImageCompression/aom,felipebetancur/libvpx,mbebenita/aom,Distrotech/libvpx,VTCSecureLLC/libvpx,Laknot/libvpx,jmvalin/aom,charup/https---github.com-webmproject-libvpx-,ShiftMediaProject/libvpx,charup/https---github.com-webmproject-libvpx-,shareefalis/libvpx,zofuthan/libvpx,goodleixiao/vpx,iniwf/webm.libvpx,smarter/aom,luctrudeau/aom,reimaginemedia/webm.libvpx,kleopatra999/webm.libvpx,smarter/aom,Suvarna1488/webm.libvpx,shyamalschandra/libvpx,felipebetancur/libvpx,kalli123/webm.libvpx,lyx2014/libvpx_c,ittiamvpx/libvpx,shyamalschandra/libvpx,reimaginemedia/webm.libvpx,shacklettbp/aom,gshORTON/webm.libvpx,kleopatra999/webm.libvpx,liqianggao/libvpx,Laknot/libvpx,kleopatra999/webm.libvpx,ShiftMediaProject/libvpx,goodleixiao/vpx,thdav/aom,goodleixiao/vpx,Maria1099/webm.libvpx,GrokImageCompression/aom,Maria1099/webm.libvpx,ittiamvpx/libvpx-1,thdav/aom,Distrotech/libvpx,iniwf/webm.libvpx,zofuthan/libvpx,altogother/webm.libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,zofuthan/libvpx,Suvarna1488/webm.libvpx,WebRTC-Labs/libvpx,mbebenita/aom,mbebenita/aom,running770/libvpx,matanbs/webm.libvpx,mwgoldsmith/vpx,jmvalin/aom,liqianggao/libvpx,smarter/aom,reimaginemedia/webm.libvpx,Acidburn0zzz/webm.libvpx,n4t/libvpx,shareefalis/libvpx,hsueceumd/test_hui,kim42083/webm.libvpx,liqianggao/libvpx,webmproject/libvpx,iniwf/webm.libvpx,n4t/libvpx,stewnorriss/libvpx,altogother/webm.libvpx,kim42083/webm.libvpx,Laknot/libvpx,thdav/aom,goodleixiao/vpx,ittiamvpx/libvpx-1,VTCSecureLLC/libvpx,pcwalton/libvpx,Maria1099/webm.libvpx,lyx2014/libvpx_c,ShiftMediaProject/libvpx,shyamalschandra/libvpx,ittiamvpx/libvpx,mbebenita/aom,n4t/libvpx,matanbs/vp982,kim42083/webm.libvpx,shyamalschandra/libvpx,stewnorriss/libvpx,n4t/libvpx,matanbs/webm.libvpx,pcwalton/libvpx,Maria1099/webm.libvpx,webmproject/libvpx,mwgoldsmith/libvpx,stewnorriss/libvpx,ittiamvpx/libvpx-1,Acidburn0zzz/webm.libvpx,matanbs/vp982,abwiz0086/webm.libvpx,ittiamvpx/libvpx,jacklicn/webm.libvpx,webmproject/libvpx,liqianggao/libvpx,hsueceumd/test_hui,jdm/libvpx,Topopiccione/libvpx,jacklicn/webm.libvpx,shyamalschandra/libvpx,smarter/aom,WebRTC-Labs/libvpx,kalli123/webm.libvpx,abwiz0086/webm.libvpx,matanbs/vp982,kleopatra999/webm.libvpx,Distrotech/libvpx,charup/https---github.com-webmproject-libvpx-,Topopiccione/libvpx,openpeer/libvpx_new,Suvarna1488/webm.libvpx,Topopiccione/libvpx,abwiz0086/webm.libvpx,reimaginemedia/webm.libvpx,smarter/aom,ittiamvpx/libvpx,hsueceumd/test_hui,Acidburn0zzz/webm.libvpx,thdav/aom,shacklettbp/aom,stewnorriss/libvpx,iniwf/webm.libvpx,VTCSecureLLC/libvpx,GrokImageCompression/aom,mwgoldsmith/libvpx,Laknot/libvpx,mwgoldsmith/vpx,charup/https---github.com-webmproject-libvpx-,matanbs/webm.libvpx,goodleixiao/vpx,Topopiccione/libvpx,charup/https---github.com-webmproject-libvpx-,jmvalin/aom,Acidburn0zzz/webm.libvpx,kalli123/webm.libvpx,jmvalin/aom,mwgoldsmith/vpx,abwiz0086/webm.libvpx,mbebenita/aom,kim42083/webm.libvpx,Maria1099/webm.libvpx,kleopatra999/webm.libvpx,hsueceumd/test_hui,jdm/libvpx,mbebenita/aom,n4t/libvpx,hsueceumd/test_hui,webmproject/libvpx,Distrotech/libvpx,gshORTON/webm.libvpx,openpeer/libvpx_new,shacklettbp/aom,Acidburn0zzz/webm.libvpx,VTCSecureLLC/libvpx,mwgoldsmith/vpx,jdm/libvpx,mwgoldsmith/libvpx,zofuthan/libvpx,lyx2014/libvpx_c,luctrudeau/aom,ittiamvpx/libvpx-1,jmvalin/aom,shareefalis/libvpx,GrokImageCompression/aom,ittiamvpx/libvpx,iniwf/webm.libvpx,gshORTON/webm.libvpx,shareefalis/libvpx,kim42083/webm.libvpx,liqianggao/libvpx,matanbs/webm.libvpx,altogother/webm.libvpx,iniwf/webm.libvpx,mwgoldsmith/libvpx,altogother/webm.libvpx,matanbs/vp982,felipebetancur/libvpx,liqianggao/libvpx,shyamalschandra/libvpx,matanbs/vp982,Topopiccione/libvpx,lyx2014/libvpx_c,hsueceumd/test_hui,pcwalton/libvpx,stewnorriss/libvpx,matanbs/vp982,ittiamvpx/libvpx-1,jdm/libvpx,jacklicn/webm.libvpx,mbebenita/aom,running770/libvpx,altogother/webm.libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,Acidburn0zzz/webm.libvpx,lyx2014/libvpx_c,matanbs/webm.libvpx,openpeer/libvpx_new,Suvarna1488/webm.libvpx,WebRTC-Labs/libvpx,gshORTON/webm.libvpx,kim42083/webm.libvpx,pcwalton/libvpx,openpeer/libvpx_new,Maria1099/webm.libvpx,running770/libvpx,running770/libvpx,matanbs/webm.libvpx,abwiz0086/webm.libvpx,luctrudeau/aom,kalli123/webm.libvpx,shareefalis/libvpx,jdm/libvpx,shacklettbp/aom,altogother/webm.libvpx,mbebenita/aom,goodleixiao/vpx,reimaginemedia/webm.libvpx,WebRTC-Labs/libvpx,pcwalton/libvpx,pcwalton/libvpx,GrokImageCompression/aom,Suvarna1488/webm.libvpx,running770/libvpx,mbebenita/aom,zofuthan/libvpx,VTCSecureLLC/libvpx,gshORTON/webm.libvpx,ittiamvpx/libvpx,smarter/aom,Laknot/libvpx,luctrudeau/aom,mwgoldsmith/vpx,kleopatra999/webm.libvpx,luctrudeau/aom,running770/libvpx,jdm/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,shareefalis/libvpx,charup/https---github.com-webmproject-libvpx-,reimaginemedia/webm.libvpx,lyx2014/libvpx_c,Suvarna1488/webm.libvpx,luctrudeau/aom,matanbs/vp982,openpeer/libvpx_new,mwgoldsmith/vpx,Laknot/libvpx,shacklettbp/aom,openpeer/libvpx_new,thdav/aom,kalli123/webm.libvpx,jmvalin/aom,felipebetancur/libvpx,stewnorriss/libvpx,Distrotech/libvpx,thdav/aom,VTCSecureLLC/libvpx,WebRTC-Labs/libvpx,jacklicn/webm.libvpx,felipebetancur/libvpx,gshORTON/webm.libvpx,jacklicn/webm.libvpx,jacklicn/webm.libvpx,kalli123/webm.libvpx