[mle] invalidate parent if not child (#4572)
[mle] invalidate parent if not child (#4572)

Current Thread certification needs Parent information even after the
node already become a router or leader. This commit sets parent state
to invalid to indicate information in mParent should not be used in
normal cases.
librasungirl/openthread,chshu/openthread,chshu/openthread,bukepo/openthread,srickardti/openthread,openthread/openthread,chshu/openthread,lanyuwen/openthread,abtink/openthread,chshu/openthread,lanyuwen/openthread,jwhui/openthread,srickardti/openthread,chshu/openthread,librasungirl/openthread,jwhui/openthread,openthread/openthread,lanyuwen/openthread,bukepo/openthread,abtink/openthread,bukepo/openthread,librasungirl/openthread,jwhui/openthread,abtink/openthread,lanyuwen/openthread,srickardti/openthread,jwhui/openthread,librasungirl/openthread,chshu/openthread,openthread/openthread,abtink/openthread,lanyuwen/openthread,bukepo/openthread,lanyuwen/openthread,openthread/openthread,srickardti/openthread