DataFlow: remove a redundant call to [CompactQueue.is_empty].

src/DataFlow.ml

@@ -206,9 +206,13 @@ module ForCustomMaps
   (* As long as the queue is nonempty, take a variable and examine it. *)
 
   let () =
-    while not (CompactQueue.is_empty pending) do
-      let var = CompactQueue.take pending in
-      B.set var false;
-      examine var
-    done
+    try
+      while true do
+        let x = CompactQueue.take pending in
+        B.set x false;
+        examine x
+      done
+    with CompactQueue.Empty ->
+      ()
+
 end