webmdec: Handle codec id being NULL.
webmdec: Handle codec id being NULL.

WebM files could have CodecId missing in the track headers. Treat those files as
unknown input file type in vpxdec.

Fixes issue #1064.

Change-Id: I6c3bb7b4bd3a4f5c244312482a5996f8b68db3f3

Topopiccione/libvpx,GrokImageCompression/aom,mbebenita/aom,Topopiccione/libvpx,mwgoldsmith/vpx,jmvalin/aom,mwgoldsmith/libvpx,webmproject/libvpx,shacklettbp/aom,Topopiccione/libvpx,ShiftMediaProject/libvpx,felipebetancur/libvpx,mwgoldsmith/vpx,Topopiccione/libvpx,openpeer/libvpx_new,webmproject/libvpx,VTCSecureLLC/libvpx,smarter/aom,mwgoldsmith/libvpx,mbebenita/aom,GrokImageCompression/aom,GrokImageCompression/aom,openpeer/libvpx_new,thdav/aom,smarter/aom,jmvalin/aom,ittiamvpx/libvpx-1,luctrudeau/aom,ittiamvpx/libvpx-1,shacklettbp/aom,mwgoldsmith/vpx,webmproject/libvpx,luctrudeau/aom,shacklettbp/aom,felipebetancur/libvpx,jmvalin/aom,Topopiccione/libvpx,smarter/aom,mbebenita/aom,ittiamvpx/libvpx-1,GrokImageCompression/aom,GrokImageCompression/aom,openpeer/libvpx_new,jmvalin/aom,mwgoldsmith/libvpx,thdav/aom,felipebetancur/libvpx,webmproject/libvpx,webmproject/libvpx,jmvalin/aom,mwgoldsmith/libvpx,openpeer/libvpx_new,openpeer/libvpx_new,openpeer/libvpx_new,mwgoldsmith/vpx,thdav/aom,thdav/aom,mbebenita/aom,shacklettbp/aom,mbebenita/aom,ShiftMediaProject/libvpx,felipebetancur/libvpx,VTCSecureLLC/libvpx,luctrudeau/aom,mbebenita/aom,smarter/aom,GrokImageCompression/aom,smarter/aom,mbebenita/aom,mwgoldsmith/vpx,mwgoldsmith/libvpx,thdav/aom,mwgoldsmith/vpx,VTCSecureLLC/libvpx,webmproject/libvpx,mwgoldsmith/libvpx,ittiamvpx/libvpx-1,mbebenita/aom,smarter/aom,ittiamvpx/libvpx-1,luctrudeau/aom,ShiftMediaProject/libvpx,VTCSecureLLC/libvpx,ittiamvpx/libvpx-1,ShiftMediaProject/libvpx,luctrudeau/aom,felipebetancur/libvpx,VTCSecureLLC/libvpx,ShiftMediaProject/libvpx,VTCSecureLLC/libvpx,luctrudeau/aom,felipebetancur/libvpx,thdav/aom,jmvalin/aom,shacklettbp/aom,Topopiccione/libvpx,mbebenita/aom,shacklettbp/aom