Fix cleanup of indirect messages when a child is removed. (#1052)
Fix cleanup of indirect messages when a child is removed. (#1052)

- Allow cleanup even when mSendBusy is true.
- Set mSendMessage to NULL to indicate that the message has been freed.
- In HandleSentFrame(), simply schedule next transmission if mSendMessage is NULL.
pvanhorn/openthread,gandreello/openthread,lanyuwen/openthread,jwhui/openthread,LeZhang2016/openthread,georgecpr/openthread,nibanks/openthread,georgecpr/openthread,xiaom-GitHub/openthread,JakubBrachTieto/openthread,nibanks/openthread,erja-gp/openthread,openthread/openthread,pvanhorn/openthread,JakubBrachTieto/openthread,Zolertia/openthread,LeZhang2016/openthread,aeliot/openthread,erja-gp/openthread,chshu/openthread,jwhui/openthread,openthread/openthread,JakubBrachTieto/openthread,georgecpr/openthread,georgecpr/openthread,LeZhang2016/openthread,gandreello/openthread,xiaom-GitHub/openthread,Zolertia/openthread,chshu/openthread,abtink/openthread,srickardti/openthread,JakubBrachTieto/openthread,vaas-krish/openthread,mszczodrak/openthread,jjlee9/openthread,LeZhang2016/openthread,xiaom-GitHub/openthread,erja-gp/openthread,vaas-krish/openthread,bukepo/openthread,GiedriusM/openthread,georgecpr/openthread,vaas-krish/openthread,Zolertia/openthread,Zolertia/openthread,GiedriusM/openthread,jjlee9/openthread,turon/openthread,jjlee9/openthread,pvanhorn/openthread,mszczodrak/openthread,nibanks/openthread,lanyuwen/openthread,mszczodrak/openthread,mszczodrak/openthread,aeliot/openthread,lanyuwen/openthread,aeliot/openthread,librasungirl/openthread,GiedriusM/openthread,jjlee3/openthread,fbsder/openthread,jwhui/openthread,lanyuwen/openthread,openthread/openthread,JakubBrachTieto/openthread,jjlee9/openthread,aeliot/openthread,bukepo/openthread,fbsder/openthread,LeZhang2016/openthread,abtink/openthread,pvanhorn/openthread,fbsder/openthread,gandreello/openthread,gandreello/openthread,turon/openthread,erja-gp/openthread,abtink/openthread,nibanks/openthread,pvanhorn/openthread,srickardti/openthread,chshu/openthread,georgecpr/openthread,GiedriusM/openthread,jjlee9/openthread,librasungirl/openthread,chshu/openthread,jjlee3/openthread,aeliot/openthread,librasungirl/openthread,abtink/openthread,bukepo/openthread,gandreello/openthread,jjlee3/openthread,chshu/openthread,Zolertia/openthread,jjlee3/openthread,vaas-krish/openthread,JakubBrachTieto/openthread,srickardti/openthread,turon/openthread,jwhui/openthread,gandreello/openthread,chshu/openthread,openthread/openthread,bukepo/openthread,aeliot/openthread,mszczodrak/openthread,erja-gp/openthread,vaas-krish/openthread,erja-gp/openthread,nibanks/openthread,fbsder/openthread,mszczodrak/openthread,LeZhang2016/openthread,xiaom-GitHub/openthread,vaas-krish/openthread,lanyuwen/openthread,jjlee9/openthread,nibanks/openthread,fbsder/openthread,fbsder/openthread,xiaom-GitHub/openthread,GiedriusM/openthread,pvanhorn/openthread,librasungirl/openthread,jjlee3/openthread,turon/openthread,lanyuwen/openthread,srickardti/openthread,xiaom-GitHub/openthread