rust: bssl-tls: Fix predicate on `established`

Update-Note: `TlsConnection::established` previously returns a handle if
the session is present. This is mostly true except when a session
resumption is configured. At this point the handshake is most likely not
truly completed. This patch aligns with the intended meaning that this
function implies.

Signed-off-by: Xiangfei Ding <xfding@google.com>
Change-Id: I3c06c692b2dc6478f55dde57e493b1226a6a6964
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/98607
Reviewed-by: David Benjamin <davidben@google.com>
Presubmit-BoringSSL-Verified: boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com>

rust/bssl-tls/src/connection/lifecycle.rs

diff --git a/rust/bssl-tls/src/connection/lifecycle.rs b/rust/bssl-tls/src/connection/lifecycle.rs
index 42aef37..658cb76 100644
--- a/rust/bssl-tls/src/connection/lifecycle.rs
+++ b/rust/bssl-tls/src/connection/lifecycle.rs
@@ -90,14 +90,7 @@
     /// Access handshake-related options if a handshake is completed and
     /// the connection is initialised.
     pub fn established<'a>(&'a mut self) -> Option<EstablishedTlsConnection<'a, R, M>> {
-        let session = unsafe {
-            // Safety: the validity of the handle `self.0` is witnessed by `self`.
-            bssl_sys::SSL_get_session(self.ptr())
-        };
-        if session.is_null() {
-            return None;
-        }
-        Some(EstablishedTlsConnection(self))
+        (!self.is_in_handshake()).then_some(EstablishedTlsConnection(self))
     }
 }