Fix multiple part uploads at once making vector size inconsistent (apache#2681)

* Fix multiple part uploads at once making vector size inconsistent

* Calculate total_parts prior to resizing the vector

Author: gruuya

object_store/src/multipart.rs

@@ -94,9 +94,9 @@ where
         if self.tasks.is_empty() {
             return Ok(());
         }
-        let total_parts = self.completed_parts.len();
         while let Poll::Ready(Some(res)) = self.tasks.poll_next_unpin(cx) {
             let (part_idx, part) = res?;
+            let total_parts = self.completed_parts.len();
             self.completed_parts
                 .resize(std::cmp::max(part_idx + 1, total_parts), None);
             self.completed_parts[part_idx] = Some(part);