[dtls] return error on multiple calls to `Dtls::Open()` (#3709)
[dtls] return error on multiple calls to `Dtls::Open()` (#3709)


turon/openthread,lanyuwen/openthread,jwhui/openthread,bukepo/openthread,turon/openthread,abtink/openthread,chshu/openthread,lanyuwen/openthread,bukepo/openthread,srickardti/openthread,bukepo/openthread,openthread/openthread,lanyuwen/openthread,librasungirl/openthread,openthread/openthread,lanyuwen/openthread,jwhui/openthread,jwhui/openthread,openthread/openthread,chshu/openthread,chshu/openthread,librasungirl/openthread,srickardti/openthread,openthread/openthread,chshu/openthread,turon/openthread,abtink/openthread,jwhui/openthread,bukepo/openthread,abtink/openthread,turon/openthread,chshu/openthread,librasungirl/openthread,lanyuwen/openthread,srickardti/openthread,abtink/openthread,lanyuwen/openthread,chshu/openthread,srickardti/openthread,librasungirl/openthread