Check WebPMuxNew return value

[radarhere]

WebPMuxNew() may return NULL - https://github.com/webmproject/libwebp/blob/3757b8afeb54e305eaef18502812a9a88b7ed662/src/webp/mux.h#L110-L116

// Creates an empty mux object.
// Returns:
//   A pointer to the newly created empty mux object.
//   Or NULL in case of memory error.
WEBP_NODISCARD static WEBP_INLINE WebPMux* WebPMuxNew(void) {
  return WebPNewInternal(WEBP_MUX_ABI_VERSION);
}