fix NPE if stop without start (#914)

Refactor the condition for the stop method to avoid
all shutdown actions if the registry hasn't been
started. Before it could result in an NPE because
the thread pool for the publisher would be null.

spectator-reg-atlas/src/main/java/com/netflix/spectator/atlas/AtlasRegistry.java

@@ -189,42 +189,42 @@ public void start() {
    * Stop the scheduler reporting Atlas data.
    */
   public void stop() {
-    // Shutdown backround tasks to collect data
-    if (scheduler != null) {
+    if (scheduler == null) {
+      logger.warn("registry stopped, but was never started");
+    } else {
+      // Shutdown background tasks to collect data
       scheduler.shutdown();
       scheduler = null;
       logger.info("stopped collecting metrics every {}ms reporting to {}", step, uri);
-    } else {
-      logger.warn("registry stopped, but was never started");
-    }
 
-    // Flush data to Atlas
-    try {
-      // Get current time at start to avoid drift while flushing
-      OverridableClock overridableClock = (OverridableClock) clock();
-      long now = clock().wallTime();
-      overridableClock.setWallTime(now); // used set time rather than underlying clock
-
-      // Data for the previous interval may not have already been written, go ahead and
-      // try to write it out
-      logger.info("flushing data for previous interval to Atlas");
-      sendToAtlas();
-
-      // Move to end of next interval and ensure it gets written out
-      logger.info("flushing data for final interval to Atlas");
-      overridableClock.setWallTime(now / lwcStepMillis * lwcStepMillis + lwcStepMillis);
-      pollMeters(overridableClock.wallTime());
-      overridableClock.setWallTime(now / stepMillis * stepMillis + stepMillis);
-      sendToAtlas();
-    } catch (Exception e) {
-      logger.warn("failed to flush data to Atlas", e);
-    }
+      // Flush data to Atlas
+      try {
+        // Get current time at start to avoid drift while flushing
+        OverridableClock overridableClock = (OverridableClock) clock();
+        long now = clock().wallTime();
+        overridableClock.setWallTime(now); // use set time rather than underlying clock
+
+        // Data for the previous interval may not have already been written, go ahead and
+        // try to write it out
+        logger.info("flushing data for previous interval to Atlas");
+        sendToAtlas();
+
+        // Move to end of next interval and ensure it gets written out
+        logger.info("flushing data for final interval to Atlas");
+        overridableClock.setWallTime(now / lwcStepMillis * lwcStepMillis + lwcStepMillis);
+        pollMeters(overridableClock.wallTime());
+        overridableClock.setWallTime(now / stepMillis * stepMillis + stepMillis);
+        sendToAtlas();
+      } catch (Exception e) {
+        logger.warn("failed to flush data to Atlas", e);
+      }
 
-    // Shutdown publisher used for sending metrics
-    try {
-      publisher.close();
-    } catch (Exception e) {
-      logger.debug("failed to cleanly shutdown publisher");
+      // Shutdown publisher used for sending metrics
+      try {
+        publisher.close();
+      } catch (Exception e) {
+        logger.debug("failed to cleanly shutdown publisher");
+      }
     }
   }