wasi-nn: deinit w/o init

[yamt]

wasi_nn_ctx_destroy seems to call backend's deinit even if init was not successful.
it doesn't seem safe at least for wasi_nn_tensorflowlite.cpp.

[lum1n0us]

Like tflite_ctx might not be initialized correctly? And then deinit_backend() will use it without validation.

[yamt]

Like tflite_ctx might not be initialized correctly? And then deinit_backend() will use it without validation.

yes.

[yamt]

fixed by #4375