Change execution order to mitigate exception effects

For unknown reasons the "firstItem" is undefined in very rare cases.
By changing the execution order, the drop will still work even if an exception occurs.

backend/src/Model/RingBuffer.ts

@@ -62,9 +62,11 @@ export class RingBuffer<T extends Lengthwise> {
   }
 
   private dropFirst() {
-    const freedSpace = this.items[this.start].length
+    const firstItem = this.items[this.start]
-    this.usage -= freedSpace
     delete this.items[this.start]
     this.start += 1
+
+    const freedSpace = firstItem.length
+    this.usage -= freedSpace
   }
 }