Ensure java.io.tmpdir is available before using it

Some environments do not have a tmpdir so it can be null.

spring-boot/src/main/java/org/springframework/boot/logging/LoggingApplicationListener.java

@@ -168,11 +168,13 @@ public class LoggingApplicationListener implements SmartApplicationListener {
 		// Logback won't read backslashes so add a clean path for it to use
 		if (!StringUtils.hasLength(System.getProperty("LOG_TEMP"))) {
 			String path = System.getProperty("java.io.tmpdir");
-			path = StringUtils.cleanPath(path);
-			if (path.endsWith("/")) {
-				path = path.substring(0, path.length() - 1);
-			}
-			System.setProperty("LOG_TEMP", path);
+            if (path!=null) {
+				path = StringUtils.cleanPath(path);
+				if (path.endsWith("/")) {
+					path = path.substring(0, path.length() - 1);
+				}
+				System.setProperty("LOG_TEMP", path);
+            }
 		}
 	}