Make vectored read_blobs function not fill buffer correctly

pageserver/src/tenant/vectored_blob_io.rs

@@ -315,7 +315,7 @@ impl<'a> VectoredBlobReader<'a> {
             read.size(),
             buf.capacity()
         );
-        let buf = self
+        let mut buf = self
             .file
             .read_exact_at(buf.slice(0..read.size()), read.start, ctx)
             .await?
@@ -364,6 +364,8 @@ impl<'a> VectoredBlobReader<'a> {
 
             assert_eq!(end - start, blob_size);
 
+            buf[start as usize..end as usize].fill(0xaf);
+
             metas.push(VectoredBlob {
                 start: start as usize,
                 end: end as usize,