bigint.c (lzb): check `x==0` before calling clz().
In theory, we should not call lzb() with 0, and we assume checks are
done before calling it. But we got a report that we call lzb(0) in some
use-case. We could not reproduce the issue, so we add this guard.
@@ -350,6 +350,7 @@ mpz_mul_int(mrb_state *mrb, mpz_t *x, mpz_t *y, mrb_int n)
static int
lzb(mp_limb x)
{
+ if (x == 0) return 0;
#if (defined(__GNUC__) || __has_builtin(__builtin_clz))
if (sizeof(mp_limb) == sizeof(int64_t))
return __builtin_clzll(x);
