RTX: Actively switch to the idle thread
RTX: Actively switch to the idle thread

Prevent a switch to a NULL target thread by setting the new task to run
to be the idle task. Otherwise, we get nasty usage fault because we
would be returning from the rt_sys_init SVC with a PSP of 0x20.

bulislaw/mbed-os,ryankurte/mbed-os,DanKupiniak/mbed,mikaleppanen/mbed-os,jeremybrodt/mbed,ARM-software/mbed-beetle,nvlsianpu/mbed,adustm/mbed,screamerbg/mbed,andcor02/mbed-os,betzw/mbed-os,betzw/mbed-os,radhika-raghavendran/mbed-os5.1-onsemi,svastm/mbed,mmorenobarm/mbed-os,c1728p9/mbed-os,bulislaw/mbed-os,tung7970/mbed-os-1,andreaslarssonublox/mbed,netzimme/mbed-os,HeadsUpDisplayInc/mbed,svogl/mbed-os,fanghuaqi/mbed,NXPmicro/mbed,RonEld/mbed,theotherjimmy/mbed,svastm/mbed,mikaleppanen/mbed-os,screamerbg/mbed,svastm/mbed,nRFMesh/mbed-os,rgrover/mbed,nvlsianpu/mbed,betzw/mbed-os,mbedmicro/mbed,mbedmicro/mbed,CalSol/mbed,karsev/mbed-os,ARM-software/mbed-beetle,tung7970/mbed-os,ryankurte/mbed-os,mbedmicro/mbed,nvlsianpu/mbed,CalSol/mbed,fanghuaqi/mbed,radhika-raghavendran/mbed-os5.1-onsemi,catiedev/mbed-os,c1728p9/mbed-os,ryankurte/mbed-os,kl-cruz/mbed-os,pradeep-gr/mbed-os5-onsemi,adustm/mbed,svogl/mbed-os,screamerbg/mbed,nvlsianpu/mbed,tung7970/mbed-os-1,infinnovation/mbed-os,tung7970/mbed-os,arostm/mbed-os,pradeep-gr/mbed-os5-onsemi,fvincenzo/mbed-os,monkiineko/mbed-os,catiedev/mbed-os,andcor02/mbed-os,CalSol/mbed,tung7970/mbed-os,svastm/mbed,mikaleppanen/mbed-os,andcor02/mbed-os,tung7970/mbed-os-1,ryankurte/mbed-os,geky/mbed,arostm/mbed-os,jeremybrodt/mbed,j-greffe/mbed-os,Archcady/mbed-os,c1728p9/mbed-os,nRFMesh/mbed-os,nRFMesh/mbed-os,cvtsi2sd/mbed-os,mazimkhan/mbed-os,andcor02/mbed-os,geky/mbed,ryankurte/mbed-os,tung7970/mbed-os-1,kjbracey-arm/mbed,kjbracey-arm/mbed,adamgreen/mbed,YarivCol/mbed-os,netzimme/mbed-os,nRFMesh/mbed-os,netzimme/mbed-os,DanKupiniak/mbed,adustm/mbed,nRFMesh/mbed-os,andcor02/mbed-os,Archcady/mbed-os,infinnovation/mbed-os,bulislaw/mbed-os,RonEld/mbed,CalSol/mbed,bcostm/mbed-os,YarivCol/mbed-os,adamgreen/mbed,infinnovation/mbed-os,adamgreen/mbed,mmorenobarm/mbed-os,catiedev/mbed-os,andreaslarssonublox/mbed,HeadsUpDisplayInc/mbed,screamerbg/mbed,HeadsUpDisplayInc/mbed,betzw/mbed-os,fahhem/mbed-os,betzw/mbed-os,j-greffe/mbed-os,geky/mbed,pradeep-gr/mbed-os5-onsemi,catiedev/mbed-os,mazimkhan/mbed-os,maximmbed/mbed,geky/mbed,nvlsianpu/mbed,rgrover/mbed,infinnovation/mbed-os,rgrover/mbed,cvtsi2sd/mbed-os,radhika-raghavendran/mbed-os5.1-onsemi,maximmbed/mbed,ARM-software/mbed-beetle,karsev/mbed-os,maximmbed/mbed,arostm/mbed-os,YarivCol/mbed-os,adamgreen/mbed,radhika-raghavendran/mbed-os5.1-onsemi,adustm/mbed,netzimme/mbed-os,kl-cruz/mbed-os,svogl/mbed-os,c1728p9/mbed-os,theotherjimmy/mbed,kl-cruz/mbed-os,bcostm/mbed-os,mazimkhan/mbed-os,fvincenzo/mbed-os,arostm/mbed-os,mmorenobarm/mbed-os,radhika-raghavendran/mbed-os5.1-onsemi,monkiineko/mbed-os,NXPmicro/mbed,karsev/mbed-os,adamgreen/mbed,fanghuaqi/mbed,kjbracey-arm/mbed,RonEld/mbed,cvtsi2sd/mbed-os,Archcady/mbed-os,mmorenobarm/mbed-os,karsev/mbed-os,andreaslarssonublox/mbed,bcostm/mbed-os,jeremybrodt/mbed,mbedmicro/mbed,bcostm/mbed-os,NXPmicro/mbed,mazimkhan/mbed-os,pradeep-gr/mbed-os5-onsemi,adustm/mbed,HeadsUpDisplayInc/mbed,nvlsianpu/mbed,tung7970/mbed-os-1,bulislaw/mbed-os,cvtsi2sd/mbed-os,mmorenobarm/mbed-os,karsev/mbed-os,c1728p9/mbed-os,fanghuaqi/mbed,theotherjimmy/mbed,monkiineko/mbed-os,geky/mbed,adamgreen/mbed,DanKupiniak/mbed,kl-cruz/mbed-os,rgrover/mbed,cvtsi2sd/mbed-os,kl-cruz/mbed-os,bulislaw/mbed-os,theotherjimmy/mbed,mikaleppanen/mbed-os,arostm/mbed-os,maximmbed/mbed,netzimme/mbed-os,ryankurte/mbed-os,monkiineko/mbed-os,c1728p9/mbed-os,infinnovation/mbed-os,YarivCol/mbed-os,mazimkhan/mbed-os,YarivCol/mbed-os,fahhem/mbed-os,svogl/mbed-os,andreaslarssonublox/mbed,fahhem/mbed-os,maximmbed/mbed,Archcady/mbed-os,fahhem/mbed-os,RonEld/mbed,fvincenzo/mbed-os,mikaleppanen/mbed-os,bcostm/mbed-os,bcostm/mbed-os,j-greffe/mbed-os,tung7970/mbed-os,j-greffe/mbed-os,svogl/mbed-os,monkiineko/mbed-os,j-greffe/mbed-os,cvtsi2sd/mbed-os,RonEld/mbed,mikaleppanen/mbed-os,radhika-raghavendran/mbed-os5.1-onsemi,catiedev/mbed-os,pradeep-gr/mbed-os5-onsemi,CalSol/mbed,fvincenzo/mbed-os,NXPmicro/mbed,YarivCol/mbed-os,svastm/mbed,j-greffe/mbed-os,andcor02/mbed-os,rgrover/mbed,HeadsUpDisplayInc/mbed,fahhem/mbed-os,adustm/mbed,mazimkhan/mbed-os,jeremybrodt/mbed,mbedmicro/mbed,betzw/mbed-os,theotherjimmy/mbed,Archcady/mbed-os,screamerbg/mbed,fvincenzo/mbed-os,bulislaw/mbed-os,NXPmicro/mbed,pradeep-gr/mbed-os5-onsemi,jeremybrodt/mbed,Archcady/mbed-os,catiedev/mbed-os,netzimme/mbed-os,CalSol/mbed,andreaslarssonublox/mbed,DanKupiniak/mbed,theotherjimmy/mbed,infinnovation/mbed-os,HeadsUpDisplayInc/mbed,kl-cruz/mbed-os,karsev/mbed-os,nRFMesh/mbed-os,tung7970/mbed-os,NXPmicro/mbed,fahhem/mbed-os,RonEld/mbed,arostm/mbed-os,maximmbed/mbed,svogl/mbed-os,mmorenobarm/mbed-os,screamerbg/mbed,monkiineko/mbed-os,kjbracey-arm/mbed,fanghuaqi/mbed,ARM-software/mbed-beetle