Fix crash when mkvparser returns a NULL BlockEntry.
Fix crash when mkvparser returns a NULL BlockEntry.

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1090

Change-Id: I64b7b19581f15d53048e3026c3f8946e344fbe26

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