Fail early in legacy exporter if master secret is not available

Fail early in legacy exporter if master secret is not available

The exporter depends on having a master secret. If the handshake is
not completed, it is neither guaranteed that a shared ciphersuite was
selected (in which case tls1_PRF() will currently NULL deref) or that
a master secret was set up (in which case the exporter will succeed
with a predictable value). Neither outcome is desirable, so error out
early instead of entering the sausage factory unprepared. This aligns
the legacy exporter with the TLSv1.3 exporter in that regard.

with/ok jsing