PVS-Studio warning V610 for PostgreSQL

static char *
inet_cidr_ntop_ipv6(const u_char *src, int bits,
                    char *dst, size_t size)
{
  ....
  m = ~0 << (8 - b);
  ....
}

This suspicious code was found in PostgreSQL project by PVS-Studio static code analyzer.
Warning message is:
V610 Undefined behavior. Check the shift operator '<<. The left operand '~0' is negative. postgres inet_cidr_ntop.c 206

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