bug#15924: [PATCH] dfa: avoid undefined behavior of "1 << 31"

[Paul Eggert]

Jim Meyering wrote:
>  static int
>  tstbit (unsigned int b, charclass const c)
>  {
> -  return c[b / INTBITS] & 1 << b % INTBITS;
> +  return c[b / INTBITS] & 1U << b % INTBITS;
>  }

On a machine with 32-bit int and where b % INTBITS is 31,
the expression c[b / INTBITS] & 1U << b % INTBITS
is of type 'unsigned' and can have the value 2**31, and
this will overflow when tstbit converts that value as an int,
leading to implementation-defined behavior, which can include
raising a signal.

Better would be something like this:

static bool
tstbit (unsigned int b, charclass const c)
{
  return c[b / INTBITS] >> b % INTBITS & 1;
}

and it'd probably be better to encourage this style in
other places where the problem occurs, e.g., quotearg.