core: provide function to check msg queue initialization
core: provide function to check msg queue initialization

This makes it easier to refactor that part of the `thread_t` structure
later on.

jasonatran/RIOT,kaspar030/RIOT,jasonatran/RIOT,BytesGalore/RIOT,ant9000/RIOT,kaspar030/RIOT,mtausig/RIOT,A-Paul/RIOT,kaspar030/RIOT,yogo1212/RIOT,authmillenon/RIOT,mtausig/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,cladmi/RIOT,ant9000/RIOT,mfrey/RIOT,mfrey/RIOT,x3ro/RIOT,OTAkeys/RIOT,smlng/RIOT,kYc0o/RIOT,miri64/RIOT,lazytech-org/RIOT,josephnoir/RIOT,basilfx/RIOT,BytesGalore/RIOT,aeneby/RIOT,rfuentess/RIOT,miri64/RIOT,kYc0o/RIOT,rfuentess/RIOT,authmillenon/RIOT,cladmi/RIOT,A-Paul/RIOT,kYc0o/RIOT,rfuentess/RIOT,BytesGalore/RIOT,x3ro/RIOT,yogo1212/RIOT,mtausig/RIOT,basilfx/RIOT,yogo1212/RIOT,cladmi/RIOT,mtausig/RIOT,lazytech-org/RIOT,josephnoir/RIOT,authmillenon/RIOT,mfrey/RIOT,yogo1212/RIOT,smlng/RIOT,toonst/RIOT,lazytech-org/RIOT,toonst/RIOT,A-Paul/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,smlng/RIOT,aeneby/RIOT,authmillenon/RIOT,mfrey/RIOT,ant9000/RIOT,cladmi/RIOT,lazytech-org/RIOT,cladmi/RIOT,kYc0o/RIOT,basilfx/RIOT,aeneby/RIOT,basilfx/RIOT,aeneby/RIOT,x3ro/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,kaspar030/RIOT,aeneby/RIOT,RIOT-OS/RIOT,toonst/RIOT,RIOT-OS/RIOT,toonst/RIOT,josephnoir/RIOT,authmillenon/RIOT,smlng/RIOT,miri64/RIOT,josephnoir/RIOT,mfrey/RIOT,OlegHahm/RIOT,lazytech-org/RIOT,A-Paul/RIOT,BytesGalore/RIOT,A-Paul/RIOT,ant9000/RIOT,jasonatran/RIOT,josephnoir/RIOT,OTAkeys/RIOT,jasonatran/RIOT,x3ro/RIOT,miri64/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,smlng/RIOT,x3ro/RIOT,rfuentess/RIOT,OlegHahm/RIOT,basilfx/RIOT,mtausig/RIOT,kYc0o/RIOT,authmillenon/RIOT,yogo1212/RIOT,BytesGalore/RIOT,miri64/RIOT,rfuentess/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,toonst/RIOT