wasm-bindgen: null pointer passed to rust

```sh
rust-c-wasm-lib/fib_wasm/pkg/fib_wasm.js:36
throw new Error(getStringFromWasm(ptr, len));
^

Error: null pointer passed to rust
at module.exports.__wbindgen_throw (/home/dvc/block/rust-c-wasm-lib/fib_wasm/pkg/fib_wasm.js:36:11)
at wasm_bindgen::throw_str::hd245d7ed4ce0494c (wasm-function[42]:5)
at wasm_bindgen::__rt::throw_null::h6a98940f13ce8bc7 (wasm-function[43]:8)
at fib_next (wasm-function[16]:97)
at Fib.next (/home/dvc/block/rust-c-wasm-lib/fib_wasm/pkg/fib_wasm.js:71:14)
at Object. (/home/dvc/block/rust-c-wasm-lib/fib_wasm/jssrc/main.js:6:19)
at Module._compile (internal/modules/cjs/loader.js:734:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:745:10)
at Module.load (internal/modules/cjs/loader.js:626:32)
at tryModuleLoad (internal/modules/cjs/loader.js:566:12)
make: *** [Makefile:2: run] Error 1
```