HBASE-24713 RS startup with FSHLog throws NPE after HBASE-21751 (#2125)

* null check for writer if not initialized yet during syncrunner run

* Revert "null check for writer if not initialized yet during syncrunner run"

This reverts commit 72932ad.

* Writer check while trying to attain safe point
Signed-off-by: Anoop Sam John <anoopsamjohn@apache.com>
Signed-off-by: Viraj Jasani <vjasani@apache.org>
Signed-off-by: Ramkrishna <ramkrishna@apache.org>

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/FSHLog.java

@@ -329,7 +329,7 @@ protected void doReplaceWriter(Path oldPath, Path newPath, Writer nextWriter) th
     SyncFuture syncFuture = null;
     SafePointZigZagLatch zigzagLatch = null;
     long sequence = -1L;
-    if (this.ringBufferEventHandler != null) {
+    if (this.writer != null && this.ringBufferEventHandler != null) {
       // Get sequence first to avoid dead lock when ring buffer is full
       // Considering below sequence
       // 1. replaceWriter is called and zigzagLatch is initialized