ARROW-8225: [Rust] Continuation marker check was in wrong location.

In the previous commit I had the continuation marker check in the wrong location.

Closes #6791 from maxburke/rust_continuation_2

Authored-by: Max Burke <max@urbanlogiq.com>
Signed-off-by: Neville Dipale <nevilledips@gmail.com>

rust/arrow/src/ipc/reader.rs

@@ -727,17 +727,22 @@ impl<R: Read> StreamReader<R> {
         // determine metadata length
         let mut meta_size: [u8; 4] = [0; 4];
         reader.read_exact(&mut meta_size)?;
-        let meta_len = u32::from_le_bytes(meta_size);
+        let meta_len = {
+            let meta_len = u32::from_le_bytes(meta_size);
+
+            // If a continuation marker is encountered, skip over it and read
+            // the size from the next four bytes.
+            if meta_len == CONTINUATION_MARKER {
+                reader.read_exact(&mut meta_size)?;
+                u32::from_le_bytes(meta_size)
+            } else {
+                meta_len
+            }
+        };
 
         let mut meta_buffer = vec![0; meta_len as usize];
         reader.read_exact(&mut meta_buffer)?;
 
-        // If a continuation marker is encountered, skip over it and read
-        // the size from the next four bytes.
-        if u32::from_le_bytes(meta_size) == CONTINUATION_MARKER {
-            reader.read_exact(&mut meta_size)?;
-        }
-
         let vecs = &meta_buffer.to_vec();
         let message = ipc::get_root_as_message(vecs);
         // message header is a Schema, so read it