Trigger call rejoin attempt only if you were in it

Many membership events may get synced while your own join attempt is
being processed, during which you will not be in the call membership
list.  So, only try to rejoin if it seems that you have _left_ the call,
not when you are not (yet) in the call.

src/matrixrtc/MatrixRTCSession.ts

@@ -871,7 +871,11 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
             logger.info(`Memberships for call in room ${this.room.roomId} have changed: emitting`);
             this.emit(MatrixRTCSessionEvent.MembershipsChanged, oldMemberships, this.memberships);
 
-            if (this.isJoined() && !this.memberships.some(this.isMyMembership)) {
+            if (
+                this.isJoined() &&
+                oldMemberships.some(this.isMyMembership) &&
+                !this.memberships.some(this.isMyMembership)
+            ) {
                 logger.warn("Missing own membership: force re-join");
                 // TODO: Should this be awaited? And is there anything to tell the focus?
                 this.triggerCallMembershipEventUpdate();