Delay DataBuffer release until stream is closed

This commit changes the release of data buffers from a finally block
into a `doFinally()` lambda on the returned `Flux<XMLEvent>` stream.

Issue: SPR-15707

spring-web/src/main/java/org/springframework/http/codec/xml/XmlEventDecoder.java

@@ -106,14 +106,14 @@ public class XmlEventDecoder extends AbstractDecoder<XMLEvent> {
 						try {
 							InputStream is = dataBuffer.asInputStream();
 							Iterator eventReader = inputFactory.createXMLEventReader(is);
-							return Flux.fromIterable((Iterable<XMLEvent>) () -> eventReader);
+							return Flux.fromIterable((Iterable<XMLEvent>) () -> eventReader)
+									.doFinally(t -> {
+										DataBufferUtils.release(dataBuffer);
+									});
 						}
 						catch (XMLStreamException ex) {
 							return Mono.error(ex);
 						}
-						finally {
-							DataBufferUtils.release(dataBuffer);
-						}
 					});
 		}
 	}