vp9_mbgraph.c: cleanup -wextra warnings
vp9_mbgraph.c: cleanup -wextra warnings

Change-Id: Ia6e2c2741adbf45f98a447dbb401506f95a2b0c8

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