PVS-Studio warning V610 for LibRaw

unsigned CLASS pana_bits (int nbits)
{
  ....
  return (buf[byte] | buf[byte+1] << 8) >>
         (vbits & 7) & ~(-1 << nbits);
  ....
}

This suspicious code was found in LibRaw project by PVS-Studio static code analyzer.
Warning message is:
V610 Undefined behavior. Check the shift operator '<<. The left operand '-1' is negative. dcraw_common.cpp 1827

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