store-core: Fixes chat commands not working
store-core: Fix missing variable tag
store-shop: Fix item categories menu

They where not shown when hide_empty_categories was disabled.
store-shop: Fix buying items not working
core, shop: Fix code using uninitialized memory

Sometimes buying items didn't work because the variable "result" wasn't initialized properly.
Merge pull request #7 from berni2288/pullrequest-bugfixes

Bugfixes for chat commands, item categories not shown, buying items