[posix-app] fix crash on os ifconfig down (#4171)
[posix-app] fix crash on os ifconfig down (#4171)

This commit fixes a crash issue triggered by system command ifconfig
down.  This is because OpenThread's tries removing addresses when the
platform network interface is already down.
librasungirl/openthread,srickardti/openthread,lanyuwen/openthread,lanyuwen/openthread,openthread/openthread,chshu/openthread,abtink/openthread,lanyuwen/openthread,srickardti/openthread,chshu/openthread,jwhui/openthread,abtink/openthread,bukepo/openthread,lanyuwen/openthread,bukepo/openthread,chshu/openthread,librasungirl/openthread,bukepo/openthread,turon/openthread,turon/openthread,srickardti/openthread,turon/openthread,chshu/openthread,srickardti/openthread,lanyuwen/openthread,bukepo/openthread,openthread/openthread,chshu/openthread,jwhui/openthread,lanyuwen/openthread,librasungirl/openthread,librasungirl/openthread,jwhui/openthread,chshu/openthread,abtink/openthread,turon/openthread,jwhui/openthread,openthread/openthread,openthread/openthread,abtink/openthread