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.

mrbgems/mruby-bigint/core/bigint.c

@@ -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);