Fix integrity error when currentAsset is undefined 🐛

[ngocphuongnb]

Description

Fix error: Cannot read property 'integrity' of undefined

Why

In case of using react-loadable-ssr-addon with dynamic imported component; when a component is removed from the project, react-loadable-ssr-addon throw error because currentAsset file is missing, we need to check currentAsset variable before getting it's integrity

my-project/node_modules/react-loadable-ssr-addon/lib/ReactLoadableSSRAddon.js:193
            integrity: currentAsset[_this.options.integrityPropertyName]
                                   ^

TypeError: Cannot read property 'integrity' of undefined
    at my-project/node_modules/react-loadable-ssr-addon/lib/ReactLoadableSSRAddon.js:193:36
    at Map.forEach (<anonymous>)
    at ReactLoadableSSRAddon.processAssets (my-project/node_modules/react-loadable-ssr-addon/lib/ReactLoadableSSRAddon.js:151:23)
    at ReactLoadableSSRAddon.handleEmit (my-project/node_modules/react-loadable-ssr-addon/lib/ReactLoadableSSRAddon.js:140:10)
    at AsyncSeriesHook.eval [as callAsync] (eval at create (my-project/node_modules/tapable/lib/HookCodeFactory.js:32:10), <anonymous>:7:1)
    at Compiler.emitAssets (my-project/node_modules/webpack/lib/Compiler.js:441:19)
    at onCompiled (my-project/node_modules/webpack/lib/Watching.js:50:19)
    at hooks.afterCompile.callAsync.err (my-project/node_modules/webpack/lib/Compiler.js:630:14)
    at AsyncSeriesHook.eval [as callAsync] (eval at create (my-project/node_modules/tapable/lib/HookCodeFactory.js:32:10), <anonymous>:6:1)
    at compilation.seal.err (my-project/node_modules/webpack/lib/Compiler.js:627:30)

[themgoncalves]

@nguyenngocphuongnb thanks for the contribution, you bough for us an interesting fix.

I just suggested another and simpler approach to this issue, could you please update as requested and test again?

[themgoncalves]

LGTM 👍