Fix clippy::assigning_clones warning

```
error: assigning the result of `Clone::clone()` may be inefficient
   --> tf_rosrust/src/tf_buffer.rs:158:21
    |
158 |                     first = intermediate.clone();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `clone_from()`: `first.clone_from(&intermediate)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assigning_clones
    = note: `-D clippy::assigning-clones` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::assigning_clones)]`
```

tf_r2r/src/tf_buffer.rs

@@ -156,7 +156,7 @@ impl TfBuffer {
                             tf_list.push(x.transform);
                         }
                     }
-                    first = intermediate.clone();
+                    first.clone_from(&intermediate);
                 }
                 let final_tf = chain_transforms(&tf_list);
                 let msg = TransformStamped {

tf_rosrust/src/tf_buffer.rs

@@ -155,7 +155,7 @@ impl TfBuffer {
                             tf_list.push(x.transform);
                         }
                     }
-                    first = intermediate.clone();
+                    first.clone_from(&intermediate);
                 }
                 let final_tf = chain_transforms(&tf_list);
                 let msg = TransformStamped {