Fix parens for AST_Undefined
Fix parens for AST_Undefined

Do the same as for AST_Unary, since we output undefined as `void 0`.

Reported at https://github.com/mishoo/UglifyJS2/issues/338#issuecomment-48858341

butchi/codedecodeJS,SpainTrain/UglifyJS2,shanaithal/UglifyJS2,microlv/UglifyJS2,githuboflee/first,vjeux/UglifyJS2,eth-srl/UnuglifyJS,kemitchell/UglifyJS2,eth-srl/UnuglifyJS,butchi/decodecodeJS,beni55/UnuglifyJS,sebmck/UglifyJS2,digitalloggers/UglifyJS2,kittens/UglifyJS2,beni55/UnuglifyJS,rvanvelzen/UglifyJS2,supriyantomaftuh/UglifyJS2,Jeremy017/UglifyJS2,GerHobbelt/UglifyJS2,dabretin/UglifyJS2,istr/UglifyJS2,VIVEKLUCKY1848/UglifyJS2,HankMcCoy/UglifyJS2,Venima/Test,sielay/UglifyJS2,hychen/UglifyJS2,avdg/UglifyJS2,jchbh-duplicate/UglifyJS2,TurkeyFried/uglifyjs2-dist,hxford/UglifyJS2,davidbau/UglifyJS2,shelsonjava/UglifyJS2