support compiler hooks afterCompile where stats.toJson is undefined

[glassdimlygr]

Fixing:

TypeError: stats.toJson is not a function
    at /Users/jjohn/www/GuaranteedRate.com-mod/node_modules/webpack-copy-on-plugin/index.js:44:33
    at AsyncSeriesHook.eval [as callAsync] (eval at create (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:9:1)
    at AsyncSeriesHook.lazyCompileHook (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/tapable/lib/Hook.js:154:20)
    at compilation.seal.err (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/webpack/lib/Compiler.js:668:31)
    at AsyncSeriesHook.eval [as callAsync] (eval at create (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:9:1)
    at AsyncSeriesHook.lazyCompileHook (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/tapable/lib/Hook.js:154:20)
    at hooks.optimizeAssets.callAsync.err (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/webpack/lib/Compilation.js:1385:35)
    at AsyncSeriesHook.eval [as callAsync] (eval at create (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:9:1)
    at AsyncSeriesHook.lazyCompileHook (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/tapable/lib/Hook.js:154:20)
    at hooks.optimizeChunkAssets.callAsync.err (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/webpack/lib/Compilation.js:1376:32)
    at AsyncSeriesHook.eval [as callAsync] (eval at create (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:9:1)
    at AsyncSeriesHook.lazyCompileHook (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/tapable/lib/Hook.js:154:20)
    at hooks.additionalAssets.callAsync.err (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/webpack/lib/Compilation.js:1371:36)
    at AsyncSeriesHook.eval [as callAsync] (eval at create (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:9:1)
    at AsyncSeriesHook.lazyCompileHook (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/tapable/lib/Hook.js:154:20)
    at hooks.optimizeTree.callAsync.err (/Users/jjohn/www/GuaranteedRate.com-mod/node_modules/webpack/lib/Compilation.js:1367:32)

The stats.toJson part of the code is for the filematch regex, rather than the simpler copyFolder functionality. This just prevents the errors.

Meets my use case, and safe to merge. But probably some deeper error reporting would be the right thing to do.