[logging] fix BeaconPayload::ToInfoString() print of network name (#3771)
[logging] fix BeaconPayload::ToInfoString() print of network name (#3771)

`BeaconPayload::GetNetworkName()` does not return a NULL-terminated string.

This commit fixes `BeaconPayload::ToInfoString()` to pass a NULL-terminated
string to the string formatter.
openthread/openthread,jwhui/openthread,jwhui/openthread,librasungirl/openthread,lanyuwen/openthread,jwhui/openthread,lanyuwen/openthread,librasungirl/openthread,turon/openthread,srickardti/openthread,abtink/openthread,bukepo/openthread,bukepo/openthread,turon/openthread,chshu/openthread,abtink/openthread,srickardti/openthread,chshu/openthread,srickardti/openthread,bukepo/openthread,abtink/openthread,chshu/openthread,jwhui/openthread,bukepo/openthread,srickardti/openthread,lanyuwen/openthread,chshu/openthread,openthread/openthread,librasungirl/openthread,openthread/openthread,openthread/openthread,lanyuwen/openthread,librasungirl/openthread,chshu/openthread,turon/openthread,abtink/openthread,chshu/openthread,lanyuwen/openthread,turon/openthread,lanyuwen/openthread