SettableTask.checkCompletingThread() only resets marker after match

Issue: SPR-15409
(cherry picked from commit 8cb24e0)

Author: jhoeller

spring-core/src/main/java/org/springframework/util/concurrent/SettableListenableFuture.java

@@ -169,7 +169,9 @@ protected void done() {
 
 		private boolean checkCompletingThread() {
 			boolean check = (this.completingThread == Thread.currentThread());
-			this.completingThread = null;  // only first check actually counts
+			if (check) {
+				this.completingThread = null;  // only first match actually counts
+			}
 			return check;
 		}
 	}