Add additional check for invalid megolm sessions

CMakeLists.txt

@@ -385,7 +385,7 @@ if(USE_BUNDLED_MTXCLIENT)
 	FetchContent_Declare(
 		MatrixClient
 		GIT_REPOSITORY https://github.com/Nheko-Reborn/mtxclient.git
-		GIT_TAG        b452a984b0fc522c21bb8df7d320bf13960974d0
+		GIT_TAG        4a598632f432953f4dbfacf6cfed4f85a1c59c5a
 		)
 	set(BUILD_LIB_EXAMPLES OFF CACHE INTERNAL "")
 	set(BUILD_LIB_TESTS OFF CACHE INTERNAL "")

io.github.NhekoReborn.Nheko.yaml

@@ -163,7 +163,7 @@ modules:
     buildsystem: cmake-ninja
     name: mtxclient
     sources:
-      - commit: b452a984b0fc522c21bb8df7d320bf13960974d0
+      - commit: 4a598632f432953f4dbfacf6cfed4f85a1c59c5a
         type: git
         url: https://github.com/Nheko-Reborn/mtxclient.git
   - config-opts:

src/Olm.cpp

@@ -1129,6 +1129,10 @@ decryptEvent(const MegolmSessionIndex &index,
     std::string msg_str;
     try {
         auto session = cache::client()->getInboundMegolmSession(index);
+        if (!session) {
+            return {DecryptionErrorCode::MissingSession, std::nullopt, std::nullopt};
+        }
+
         auto sessionData =
           cache::client()->getMegolmSessionData(index).value_or(GroupSessionData{});