Fix potential dobule-borrow panic. (#3807)

There is one place in code, where we can potentially panic on double borrow, because the drop routine is done while having data borrowed.

# Important Notes
The issue was easily reproducible on @Frizi machine.

lib/rust/frp/src/network.rs

@@ -193,7 +193,10 @@ impl BridgeNetwork {
     }
 
     fn destroy(&self) {
-        *self.data.borrow_mut() = None
+        self.data.take();
+        // Beware: doing it in another, also intuitive way
+        // *self.data.borrow_mut() = None
+        // May cause a panic, because the drop procedure is done while keeping data borrowed.
     }
 }