Javascript calls to wasm version of libzim/kiwix-lib only work the first time

[mossroy]

This is a follow-up of #116.

After the first call, all subsequent calls fail with the following error in the browser console :
Assertion failed: you need to wait for the runtime to be ready (e.g. wait for main() to be called)

There's certainly something wrong in the way I implemented that, but the runtime was ready for the first working call.

Here is how to reproduce :

• download https://download.kiwix.org/zim/wikipedia/wikipedia_en_ray_charles_2018-10.zim locally
• open https://mossroy.github.io/libzim_wasm/ (or index.html from https://github.com/mossroy/libzim_wasm, accessed through a local webserver or through https) in a browser that supports WebAssembly (tested on Firefox 67 and Chromium 73), and open its console
• select the downloaded ZIM file
• check that runtime initialized appears in the console (it's triggered by Module['onRuntimeInitialized'])
• click on "Read article count (with kiwix-lib)" button
• the article count is read in the ZIM file through the wasm version of libzim
• click again on the same button : it fails with the mentioned error in the console

The bindings are currently implemented with embind, in file demo_file_api.cpp. This test case only uses the getArticleCount function (same name in C and javascript).

If one wants to modify and recompile the C code, it can be done with make (see the provided Makefile, you'll need emscripten and a few libraries) or ran inside a docker container (that already contains everything). See https://github.com/mossroy/libzim_wasm/blob/master/README.md for how to do it.

[mossroy]

This seems to be improved with version 2.0.32 of emscripten.
It now works to call several times the libzim APIs, which is cool.
But there is each time an error message in the browser console :

Uncaught RuntimeError: Aborted(Module.thisProgram has been replaced with plain thisProgram (the initial value can be provided on Module, but after startup the value is only looked for on a local variable of that name))

So there is probably something to fix in our code anyway

[mossroy]

I could fix this by changing the lifecycle of the wasm module : it's (re)-initialized only when the user chooses another ZIM file(s)