crypto: Simplify the Olm message map construction.

matrix_sdk_crypto/src/machine.rs

@@ -1080,13 +1080,10 @@ impl OlmMachine {
                     Err(e) => return Err(e),
                 };
 
-                if !messages.contains_key(device.user_id()) {
-                    messages.insert(device.user_id().clone(), BTreeMap::new());
-                };
-
-                let user_messages = messages.get_mut(device.user_id()).unwrap();
-
-                user_messages.insert(
+                messages
+                    .entry(device.user_id().clone())
+                    .or_insert(BTreeMap::new())
+                    .insert(
                         DeviceIdOrAllDevices::DeviceId(device.device_id().into()),
                         serde_json::value::to_raw_value(&encrypted)?,
                     );