Safe access to 'src' of the last "script" tag

[ilyazub]

There is an error in dev-tools on first page load.

Cannot read property 'src' of undefined
    at getCurrentScriptUrl (hotModuleReplacement.js:17)
    at webpackJsonp.../node_modules/css-hot-loader/hotModuleReplacement.js.module.exports (hotModuleReplacement.js:93)

webpack-dev-server triggers rebuild right after first bundle without any changes in sources. This issue with webpack-dev-server is covered many times, most detailed is webpack/watchpack#25.
Full load of heavy-weighted SPA (even in development environment) doesn't finish before rebuild, module.hot.dispose is called, error is thrown.

This PR works around webpack/watchpack#25 for css-hot-loader.

[coveralls]

Coverage decreased (-0.3%) to 82.418% when pulling 701af6c on ilyazub:safe_access_to_script_src into 1fa44f3 on shepherdwind:master.

[ilyazub]

@shepherdwind Should I add tests for that case or we just skip 0.3% coverage decrease.

[shepherdwind]

I think this case don't need test, this coverage decrease can ignore.

[ilyazub]

May you merge this pull request?

[shepherdwind]

v1.3.6

[shepherdwind]

Thanks for you pr.