left shift of negative value in mc_chroma_unscaled()

[tsmith]

Summary of the bug:
UBSan: libavcodec/vp9.c:2758:44: runtime error: left shift of negative value -23

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
Input #0, ivf, from '/home/user/Desktop/vpx/corpus/bb6b5274722c3805b0dc5a19d48291090d050f1f':
  Duration: 00:18:12.33, bitrate: N/A
    Stream #0:0: Video: vp9 (Profile 0) (VP90 / 0x30395056), yuv420p(tv), 320x240, 30 tbr, 30 tbn, 30 tbc
[wrapped_avframe @ 0x619000003780] Warning: not compiled with thread support, using thread emulation
[vp9 @ 0x619000004180] Warning: not compiled with thread support, using thread emulation
Output #0, null, to 'pipe:':
  Metadata:
    encoder         : Lavf57.21.101
    Stream #0:0: Video: wrapped_avframe, yuv420p, 320x240, q=2-31, 200 kb/s, 30 fps, 30 tbn, 30 tbc
    Metadata:
      encoder         : Lavc57.22.100 wrapped_avframe
Stream mapping:
  Stream #0:0 -> #0:0 (vp9 (native) -> wrapped_avframe (native))
Press [q] to stop, [?] for help
libavcodec/vp9.c:2758:44: runtime error: left shift of negative value -23
    #0 0x1e09c53 in mc_chroma_unscaled /home/user/code/ffmpeg/libavcodec/vp9.c:2769:34
    #1 0x1e09c53 in inter_pred_8bpp /home/user/code/ffmpeg/libavcodec/vp9_mc_template.c:427
    #2 0x1ca28a7 in inter_recon /home/user/code/ffmpeg/libavcodec/vp9.c:2986:13
    #3 0x1ca28a7 in inter_recon_8bpp /home/user/code/ffmpeg/libavcodec/vp9.c:3040
    #4 0x1ca28a7 in decode_b /home/user/code/ffmpeg/libavcodec/vp9.c:3294
    #5 0x1c901e5 in decode_sb /home/user/code/ffmpeg/libavcodec/vp9.c:3392:17
    #6 0x1c75f55 in vp9_decode_frame /home/user/code/ffmpeg/libavcodec/vp9.c:4164:29
    #7 0x1a3fc96 in avcodec_decode_video2 /home/user/code/ffmpeg/libavcodec/utils.c:2107:19
    #8 0x589546 in decode_video /home/user/code/ffmpeg/ffmpeg.c:2069:11
    #9 0x589546 in process_input_packet /home/user/code/ffmpeg/ffmpeg.c:2318
    #10 0x5990bc in process_input /home/user/code/ffmpeg/ffmpeg.c:3980:5
    #11 0x5726c6 in transcode_step /home/user/code/ffmpeg/ffmpeg.c:4068:11
    #12 0x5726c6 in transcode /home/user/code/ffmpeg/ffmpeg.c:4122
    #13 0x56f73c in main /home/user/code/ffmpeg/ffmpeg.c:4314:9
    #14 0x7f30cf98eec4 in __libc_start_main /build/buildd/eglibc-2.19/csu/libc-start.c:287
    #15 0x466445 in _start (/home/user/Desktop/ffmpeg/ffmpeg_full+0x466445)

[tsmith]

Dup of 5127. Sorry.