Fix a crash in vpxdec
Fix a crash in vpxdec

If the codec fails to return a frame on the second frame (as in a VP8 alt-ref,
then img will be a null pointer and dereferencing it while trying to write
the color format (again) will crash.

Change-Id: Ie48710d4c5715f532d453c879c71c20ccdeb81cc

jdm/libvpx,matanbs/webm.libvpx,goodleixiao/vpx,luctrudeau/aom,running770/libvpx,GrokImageCompression/aom,altogother/webm.libvpx,running770/libvpx,zofuthan/libvpx,smarter/aom,kim42083/webm.libvpx,GrokImageCompression/aom,kleopatra999/webm.libvpx,Laknot/libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,Suvarna1488/webm.libvpx,ittiamvpx/libvpx,gshORTON/webm.libvpx,VTCSecureLLC/libvpx,felipebetancur/libvpx,jdm/libvpx,mbebenita/aom,webmproject/libvpx,Distrotech/libvpx,Suvarna1488/webm.libvpx,jmvalin/aom,mwgoldsmith/vpx,webmproject/libvpx,matanbs/vp982,Maria1099/webm.libvpx,goodleixiao/vpx,luctrudeau/aom,abwiz0086/webm.libvpx,hsueceumd/test_hui,turbulenz/libvpx,matanbs/vp982,altogother/webm.libvpx,turbulenz/libvpx,Distrotech/libvpx,ittiamvpx/libvpx-1,ittiamvpx/libvpx,jmvalin/aom,shareefalis/libvpx,jdm/libvpx,mwgoldsmith/libvpx,zofuthan/libvpx,Acidburn0zzz/webm.libvpx,shacklettbp/aom,openpeer/libvpx_new,Laknot/libvpx,kleopatra999/webm.libvpx,stewnorriss/libvpx,ittiamvpx/libvpx-1,gshORTON/webm.libvpx,matanbs/webm.libvpx,Suvarna1488/webm.libvpx,mbebenita/aom,jdm/libvpx,iniwf/webm.libvpx,thdav/aom,pcwalton/libvpx,shareefalis/libvpx,kim42083/webm.libvpx,goodleixiao/vpx,smarter/aom,Laknot/libvpx,kleopatra999/webm.libvpx,matanbs/vp982,altogother/webm.libvpx,jdm/libvpx,cinema6/libvpx,thdav/aom,liqianggao/libvpx,kim42083/webm.libvpx,jacklicn/webm.libvpx,pcwalton/libvpx,jdm/libvpx,running770/libvpx,gshORTON/webm.libvpx,VTCSecureLLC/libvpx,Suvarna1488/webm.libvpx,GrokImageCompression/aom,GrokImageCompression/aom,sanyaade-teachings/libvpx,n4t/libvpx,mwgoldsmith/vpx,openpeer/libvpx_new,jacklicn/webm.libvpx,openpeer/libvpx_new,matanbs/webm.libvpx,ShiftMediaProject/libvpx,WebRTC-Labs/libvpx,abwiz0086/webm.libvpx,WebRTC-Labs/libvpx,luctrudeau/aom,shacklettbp/aom,turbulenz/libvpx,reimaginemedia/webm.libvpx,reimaginemedia/webm.libvpx,running770/libvpx,Distrotech/libvpx,lyx2014/libvpx_c,Topopiccione/libvpx,smarter/aom,hsueceumd/test_hui,shareefalis/libvpx,Topopiccione/libvpx,matanbs/vp982,jmvalin/aom,kim42083/webm.libvpx,mwgoldsmith/vpx,VTCSecureLLC/libvpx,iniwf/webm.libvpx,mbebenita/aom,Distrotech/libvpx,turbulenz/libvpx,GrokImageCompression/aom,shacklettbp/aom,hsueceumd/test_hui,smarter/aom,kleopatra999/webm.libvpx,reimaginemedia/webm.libvpx,mbebenita/aom,Distrotech/libvpx,cinema6/libvpx,VTCSecureLLC/libvpx,charup/https---github.com-webmproject-libvpx-,hsueceumd/test_hui,zofuthan/libvpx,webmproject/libvpx,jmvalin/aom,kalli123/webm.libvpx,charup/https---github.com-webmproject-libvpx-,kleopatra999/webm.libvpx,shareefalis/libvpx,kim42083/webm.libvpx,liqianggao/libvpx,ShiftMediaProject/libvpx,sanyaade-teachings/libvpx,Topopiccione/libvpx,charup/https---github.com-webmproject-libvpx-,felipebetancur/libvpx,sanyaade-teachings/libvpx,altogother/webm.libvpx,goodleixiao/vpx,charup/https---github.com-webmproject-libvpx-,zofuthan/libvpx,pcwalton/libvpx,hsueceumd/test_hui,matanbs/webm.libvpx,luctrudeau/aom,shareefalis/libvpx,lyx2014/libvpx_c,lyx2014/libvpx_c,cinema6/libvpx,Laknot/libvpx,thdav/aom,liqianggao/libvpx,mwgoldsmith/libvpx,pcwalton/libvpx,stewnorriss/libvpx,reimaginemedia/webm.libvpx,sanyaade-teachings/libvpx,pcwalton/libvpx,Acidburn0zzz/webm.libvpx,iniwf/webm.libvpx,Laknot/libvpx,ittiamvpx/libvpx,running770/libvpx,shyamalschandra/libvpx,VTCSecureLLC/libvpx,Suvarna1488/webm.libvpx,jacklicn/webm.libvpx,cinema6/libvpx,kim42083/webm.libvpx,ittiamvpx/libvpx,webmproject/libvpx,felipebetancur/libvpx,iniwf/webm.libvpx,jacklicn/webm.libvpx,ittiamvpx/libvpx-1,mbebenita/aom,thdav/aom,mwgoldsmith/libvpx,Maria1099/webm.libvpx,abwiz0086/webm.libvpx,matanbs/vp982,shyamalschandra/libvpx,kalli123/webm.libvpx,matanbs/webm.libvpx,matanbs/webm.libvpx,hsueceumd/test_hui,mwgoldsmith/vpx,reimaginemedia/webm.libvpx,zofuthan/libvpx,turbulenz/libvpx,sanyaade-teachings/libvpx,stewnorriss/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,ittiamvpx/libvpx-1,mbebenita/aom,ittiamvpx/libvpx-1,felipebetancur/libvpx,kalli123/webm.libvpx,smarter/aom,Acidburn0zzz/webm.libvpx,jmvalin/aom,iniwf/webm.libvpx,WebRTC-Labs/libvpx,jacklicn/webm.libvpx,zofuthan/libvpx,shacklettbp/aom,goodleixiao/vpx,abwiz0086/webm.libvpx,charup/https---github.com-webmproject-libvpx-,lyx2014/libvpx_c,ittiamvpx/libvpx,Acidburn0zzz/webm.libvpx,matanbs/vp982,Maria1099/webm.libvpx,charup/https---github.com-webmproject-libvpx-,Distrotech/libvpx,n4t/libvpx,kalli123/webm.libvpx,jacklicn/webm.libvpx,shyamalschandra/libvpx,turbulenz/libvpx,Topopiccione/libvpx,n4t/libvpx,matanbs/vp982,liqianggao/libvpx,Topopiccione/libvpx,liqianggao/libvpx,shacklettbp/aom,shacklettbp/aom,Acidburn0zzz/webm.libvpx,abwiz0086/webm.libvpx,smarter/aom,shareefalis/libvpx,shyamalschandra/libvpx,openpeer/libvpx_new,Acidburn0zzz/webm.libvpx,gshORTON/webm.libvpx,Maria1099/webm.libvpx,kalli123/webm.libvpx,shyamalschandra/libvpx,jmvalin/aom,ittiamvpx/libvpx-1,stewnorriss/libvpx,turbulenz/libvpx,lyx2014/libvpx_c,mbebenita/aom,mwgoldsmith/libvpx,webmproject/libvpx,gshORTON/webm.libvpx,WebRTC-Labs/libvpx,thdav/aom,stewnorriss/libvpx,ShiftMediaProject/libvpx,altogother/webm.libvpx,n4t/libvpx,running770/libvpx,Laknot/libvpx,cinema6/libvpx,mbebenita/aom,goodleixiao/vpx,cinema6/libvpx,openpeer/libvpx_new,turbulenz/libvpx,kleopatra999/webm.libvpx,pcwalton/libvpx,felipebetancur/libvpx,Maria1099/webm.libvpx,cinema6/libvpx,openpeer/libvpx_new,mwgoldsmith/libvpx,luctrudeau/aom,Suvarna1488/webm.libvpx,mbebenita/aom,thdav/aom,kalli123/webm.libvpx,abwiz0086/webm.libvpx,stewnorriss/libvpx,shyamalschandra/libvpx,luctrudeau/aom,felipebetancur/libvpx,reimaginemedia/webm.libvpx,mwgoldsmith/vpx,turbulenz/libvpx,liqianggao/libvpx,GrokImageCompression/aom,VTCSecureLLC/libvpx,webmproject/libvpx,iniwf/webm.libvpx,lyx2014/libvpx_c,WebRTC-Labs/libvpx,Topopiccione/libvpx,n4t/libvpx,gshORTON/webm.libvpx,altogother/webm.libvpx,ittiamvpx/libvpx,Maria1099/webm.libvpx