sys/posix/pthread/pthread.c: check reaper creation
sys/posix/pthread/pthread.c: check reaper creation

Checking whether or not the pthread reaper was created
And handeling the error when it was not created

yogo1212/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,miri64/RIOT,OTAkeys/RIOT,toonst/RIOT,yogo1212/RIOT,jasonatran/RIOT,ant9000/RIOT,OTAkeys/RIOT,mtausig/RIOT,kYc0o/RIOT,kaspar030/RIOT,josephnoir/RIOT,miri64/RIOT,basilfx/RIOT,authmillenon/RIOT,OlegHahm/RIOT,yogo1212/RIOT,basilfx/RIOT,authmillenon/RIOT,x3ro/RIOT,aeneby/RIOT,x3ro/RIOT,yogo1212/RIOT,ant9000/RIOT,kYc0o/RIOT,smlng/RIOT,kaspar030/RIOT,toonst/RIOT,basilfx/RIOT,RIOT-OS/RIOT,mtausig/RIOT,kYc0o/RIOT,mtausig/RIOT,aeneby/RIOT,josephnoir/RIOT,smlng/RIOT,mtausig/RIOT,OlegHahm/RIOT,toonst/RIOT,authmillenon/RIOT,OlegHahm/RIOT,aeneby/RIOT,x3ro/RIOT,yogo1212/RIOT,ant9000/RIOT,smlng/RIOT,jasonatran/RIOT,authmillenon/RIOT,basilfx/RIOT,kaspar030/RIOT,ant9000/RIOT,aeneby/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kaspar030/RIOT,OTAkeys/RIOT,toonst/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,josephnoir/RIOT,toonst/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,miri64/RIOT,mtausig/RIOT,x3ro/RIOT,authmillenon/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,OlegHahm/RIOT,miri64/RIOT,basilfx/RIOT,OlegHahm/RIOT,authmillenon/RIOT,ant9000/RIOT,smlng/RIOT,x3ro/RIOT,OTAkeys/RIOT,aeneby/RIOT,kYc0o/RIOT,josephnoir/RIOT,yogo1212/RIOT,smlng/RIOT