Fix checking for whether segment exists on a frozen in-memory layer.

Ever since we've had frozen in-memory layers, having an 'end_lsn' no
longer means that the layer has been dropped. Need to check the 'dropped'
flag explicitly.

This was reliably causing a failure on the new 'test_parallel_copy' test
in https://github.com/zenithdb/zenith/pull/864. I'm not sure why it
doesn't happen on main branch, but the bug is pretty straightforward when
you see it.

pageserver/src/layered_repository/inmemory_layer.rs

@@ -247,9 +247,13 @@ impl Layer for InMemoryLayer {
         assert!(lsn >= self.start_lsn);
 
         // Is the requested LSN after the segment was dropped?
-        if let Some(end_lsn) = inner.end_lsn {
-            if lsn >= end_lsn {
-                return Ok(false);
+        if inner.dropped {
+            if let Some(end_lsn) = inner.end_lsn {
+                if lsn >= end_lsn {
+                    return Ok(false);
+                }
+            } else {
+                panic!("dropped in-memory layer with no end LSN");
             }
         }