Prevent UB on memcpy and floating point conversions

- add `memcpy_no_ub` that accepts null pointers for 0 count
- prevent 0 length allocation in `js_worker_postMessage`
- use safer test for `int` value in `JS_NewFloat64`,
  `JS_ToArrayLengthFree` and `js_typed_array_indexOf`

libbf.c

@@ -309,7 +309,7 @@ int bf_set(bf_t *r, const bf_t *a)
     }
     r->sign = a->sign;
     r->expn = a->expn;
-    memcpy(r->tab, a->tab, a->len * sizeof(limb_t));
+    memcpy_no_ub(r->tab, a->tab, a->len * sizeof(limb_t));
     return 0;
 }