PVS-Studio warning V610 for speex

EXPORT int speex_bits_unpack_signed(SpeexBits *bits,
                                    int nbBits)
{
   unsigned int d=speex_bits_unpack_unsigned(bits,nbBits);
   /* If number is negative */
   if (d>>(nbBits-1))
   {
      d |= (-1)<<nbBits;
   }
   return d;
}

This suspicious code was found in speex project by PVS-Studio static code analyzer.
Warning message is:
V610 Undefined behavior. Check the shift operator '<<. The left operand '(- 1)' is negative. bits.c 278

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/