vp8,GetSigned: silence unsigned int overflow warning
vp8,GetSigned: silence unsigned int overflow warning

in non-conformant fuzzed bitstreams the calculation of br->value may
overflow. this is defined behavior and harmless in that the stream is
already corrupt.

BUG=b/148271109

Change-Id: I3668ada57e0bd68cea86b82917fb03c19ac1283d

webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx