Bookie translate to readonly mode due to NullPointerException while flushing mem table.

**BUG REPORT**
When flushing mem table, the EntryMemTable.flush() method throws an NullPointerException and it cause the Bookie translate into readonly mode, and never recover to R/W mode.
***Describe the bug***
While processing FileInfo.readAbsolute() method , it passed the first check (fc == null) but got NullPointerException while doing fc.read(bb.start), we think the fc may be closed during two synchronized lock, and that cause the problem.

ERROR LOG:
```
2024-04-04 04:26:10,349Z [SortedLedgerStorage-0] ERROR org.apache.bookkeeper.bookie.SortedLedgerStorage - Exception thrown while flushing skip list cache.
java.lang.NullPointerException: Cannot invoke "java.nio.channels.FileChannel.read(java.nio.ByteBuffer, long)" because "this.fc" is null
at org.apache.bookkeeper.bookie.FileInfo.readAbsolute(FileInfo.java:426) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
at org.apache.bookkeeper.bookie.FileInfo.read(FileInfo.java:396) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
at org.apache.bookkeeper.bookie.LedgerEntryPage.readPage(LedgerEntryPage.java:196) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
at org.apache.bookkeeper.bookie.IndexPersistenceMgr.updatePage(IndexPersistenceMgr.java:650) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
at org.apache.bookkeeper.bookie.IndexInMemPageMgr.grabLedgerEntryPage(IndexInMemPageMgr.java:449) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
at org.apache.bookkeeper.bookie.IndexInMemPageMgr.getLedgerEntryPage(IndexInMemPageMgr.java:414) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
at org.apache.bookkeeper.bookie.IndexInMemPageMgr.putEntryOffset(IndexInMemPageMgr.java:573) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
at org.apache.bookkeeper.bookie.LedgerCacheImpl.putEntryOffset(LedgerCacheImpl.java:107) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
at org.apache.bookkeeper.bookie.InterleavedLedgerStorage.processEntry(InterleavedLedgerStorage.java:539) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
at org.apache.bookkeeper.bookie.SortedLedgerStorage.process(SortedLedgerStorage.java:288) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
at org.apache.bookkeeper.bookie.EntryMemTable.flushSnapshot(EntryMemTable.java:255) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
at org.apache.bookkeeper.bookie.EntryMemTable.flush(EntryMemTable.java:205) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
timed out waiting for input: auto-logoutgerStorage.java:317) ~[org.apache.bookkeeper-bookkeeper-server-4.14.3]
Connection closed.il.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:?]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:?]

```

***To Reproduce***
It difficult to reproduce, happen occasionally.

***Additional context***
We want to add a second check if the fc is null in second synchronized, and if the fc equals null ,we clear the ByteBuffer and return 0. However, we don't know whether this change will cause another problem or is there a better way to fix this bug.
org.apache.bookkeeper.bookie.FileInfo#readAbsolute
```
/**
* Read data from position start to fill the byte buffer bb.
* If bestEffort is provided, it would return when it reaches EOF.
* Otherwise, it would throw {@link org.apache.bookkeeper.bookie.ShortReadException}
* if it reaches EOF.
*
* @param bb
* byte buffer of data
* @param start
* start position to read data
* @param bestEffort
* flag indicates if it is a best-effort read
* @return number of bytes read
* @throws IOException
*/
private int readAbsolute(ByteBuffer bb, long start, boolean bestEffort)
throws IOException {
checkOpen(false);
synchronized (this) {
if (fc == null) {
return 0;
}
}
int total = 0;
int rc = 0;
while (bb.remaining() > 0) {
synchronized (this) {
// fix code start
if (null == fc){
bb.clear();
return 0;
}
// fix code end
rc = fc.read(bb, start);
}
if (rc <= 0) {
if (bestEffort) {
return total;
} else {
throw new ShortReadException("Short read at " + getLf().getPath() + "@" + start);
}
}
total += rc;
// should move read position
start += rc;
}
return total;
}
```