left shift of negative value in decode_frame_header()

[tsmith]

Summary of the bug:
UBSan: libavcodec/vp9.c:732:64: runtime error: left shift of negative value

How to reproduce:

% ffmpeg -f ivf -i <test_case> -f null -
ffmpeg version N-77718-g72673ad Copyright (c) 2000-2016 the FFmpeg developers
  built with Ubuntu clang version 3.7.1-svn253742-1~exp1 (branches/release_37) (based on LLVM 3.7.1)
  configuration: --cc=clang --cxx=clang++ --disable-libxcb --disable-xlib --disable-logging --disable-ffprobe --disable-ffplay --disable-sdl --disable-ffserver --disable-doc --disable-pthreads --disable-network --disable-d3d11va --disable-dxva2 --disable-vaapi --disable-vda --disable-vdpau --disable-stripping --disable-runtime-cpudetect --disable-securetransport --disable-iconv
  libavutil      55. 12.100 / 55. 12.100
  libavcodec     57. 22.100 / 57. 22.100
  libavformat    57. 21.101 / 57. 21.101
  libavdevice    57.  0.100 / 57.  0.100
  libavfilter     6. 23.100 /  6. 23.100
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.101 /  2.  0.101
[vp9 @ 0x619000005a80] Warning: not compiled with thread support, using thread emulation
libavcodec/vp9.c:732:64: runtime error: left shift of negative value -53
    #0 0x1c7f89c in decode_frame_header /home/user/code/ffmpeg/libavcodec/vp9.c:732:64
    #1 0x1c72f6c in vp9_decode_frame /home/user/code/ffmpeg/libavcodec/vp9.c:3970:16
    #2 0x1a3fc96 in avcodec_decode_video2 /home/user/code/ffmpeg/libavcodec/utils.c:2107:19
    #3 0xc27c95 in try_decode_frame /home/user/code/ffmpeg/libavformat/utils.c:2760:19
    #4 0xc1f630 in avformat_find_stream_info /home/user/code/ffmpeg/libavformat/utils.c:3412:9
    #5 0x53c6cb in open_input_file /home/user/code/ffmpeg/ffmpeg_opt.c:970:11
    #6 0x53a94f in open_files /home/user/code/ffmpeg/ffmpeg_opt.c:2999:15
    #7 0x53a11c in ffmpeg_parse_options /home/user/code/ffmpeg/ffmpeg_opt.c:3036:11
    #8 0x56f5ab in main /home/user/code/ffmpeg/ffmpeg.c:4292:11
    #9 0x7f1fc1270ec4 in __libc_start_main /build/buildd/eglibc-2.19/csu/libc-start.c:287
    #10 0x466445 in _start (/home/user/Desktop/ffmpeg/ffmpeg_full+0x466445)

[Ronald S. Bultje]

Fixed in b86339a9f8d7cf86995e89c7d6683ce3f64a7525.