[mle] ensure Link Metrics methods free newly allocated msg on error (#6889)
[mle] ensure Link Metrics methods free newly allocated msg on error (#6889)

This commit fixes methods `SendLinkMetricsManagementResponse()`,
`SendLinkMetricsManagementRequest()` and `SendLinkProbe()` in `Mle`
to ensure that the allocated MLE message is freed on an error. Note
that on success the `Mle::SendMessage()` will take the ownership of
the allocated `Message`.
jwhui/openthread,srickardti/openthread,srickardti/openthread,openthread/openthread,openthread/openthread,jwhui/openthread,openthread/openthread,abtink/openthread,jwhui/openthread,abtink/openthread,abtink/openthread,srickardti/openthread,abtink/openthread,jwhui/openthread,srickardti/openthread,openthread/openthread