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`

cutils.c

@@ -140,7 +140,7 @@ int dbuf_put(DynBuf *s, const uint8_t *data, size_t len)
         if (dbuf_realloc(s, s->size + len))
             return -1;
     }
-    memcpy(s->buf + s->size, data, len);
+    memcpy_no_ub(s->buf + s->size, data, len);
     s->size += len;
     return 0;
 }