Fix propagation of local.mongo.port up the context hierarchy

Previously, a StackOverflowError would occur when using a random port
for embedded mongo as the logic for propagating the property up the
context hierarchy would repeatedly use the leaf context's parent.

This commit updates the logic to look to see if the current context
has a parent, only calling the method again if it does.

Closes gh-3956

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/mongo/embedded/EmbeddedMongoAutoConfiguration.java

@@ -173,8 +173,8 @@ public class EmbeddedMongoAutoConfiguration {
 			}
 			map.put("local.mongo.port", port);
 		}
-		if (this.context.getParent() != null) {
-			setPortProperty(this.context.getParent(), port);
+		if (context.getParent() != null) {
+			setPortProperty(context.getParent(), port);
 		}
 	}