Fix single-check idiom in UnmodifiableMultiValueMap

Read the respective fields only once in the values(), entrySet(), and
keySet() methods.

Closes gh-35822

Signed-off-by: Patrick Strawderman <pstrawderman@netflix.com>

(cherry picked from commit 3b6be3d)

Author: kilink

spring-core/src/main/java/org/springframework/util/UnmodifiableMultiValueMap.java

@@ -145,26 +145,32 @@ public String toString() {
 
 	@Override
 	public Set<K> keySet() {
-		if (this.keySet == null) {
-			this.keySet = Collections.unmodifiableSet(this.delegate.keySet());
+		Set<K> keySet = this.keySet;
+		if (keySet == null) {
+			keySet = Collections.unmodifiableSet(this.delegate.keySet());
+			this.keySet = keySet;
 		}
-		return this.keySet;
+		return keySet;
 	}
 
 	@Override
 	public Set<Entry<K, List<V>>> entrySet() {
-		if (this.entrySet == null) {
-			this.entrySet = new UnmodifiableEntrySet<>(this.delegate.entrySet());
+		Set<Entry<K, List<V>>> entrySet = this.entrySet;
+		if (entrySet == null) {
+			entrySet = new UnmodifiableEntrySet<>(this.delegate.entrySet());
+			this.entrySet = entrySet;
 		}
-		return this.entrySet;
+		return entrySet;
 	}
 
 	@Override
 	public Collection<List<V>> values() {
-		if (this.values == null) {
-			this.values = new UnmodifiableValueCollection<>(this.delegate.values());
+		Collection<List<V>> values = this.values;
+		if (values == null) {
+			values = new UnmodifiableValueCollection<>(this.delegate.values());
+			this.values = values;
 		}
-		return this.values;
+		return values;
 	}
 
 	// unsupported