detect incomplete commitlogheader

Kelvin reported:

I just came across a corrupted CL file. Here's the stacktrace when starting the server:
Listening for transport dt_socket at address: 8888
java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.apache.commons.daemon.support.DaemonLoader.load(DaemonLoader.java:160)
Caused by: java.io.EOFException
at java.io.RandomAccessFile.readInt(RandomAccessFile.java:725)
at java.io.RandomAccessFile.readLong(RandomAccessFile.java:758)
at org.apache.cassandra.db.commitlog.CommitLogHeader.readCommitLogHeader(CommitLogHeader.java:145)
at org.apache.cassandra.db.commitlog.CommitLog.recover(CommitLog.java:181)
at org.apache.cassandra.db.commitlog.CommitLog.recover(CommitLog.java:167)
at org.apache.cassandra.thrift.CassandraDaemon.setup(CassandraDaemon.java:95)
at org.apache.cassandra.thrift.CassandraDaemon.init(CassandraDaemon.java:142)
... 5 more

He added that the segment is only 6 bytes long, indicating that the header was never completely written. CLH should catch that EOF and skip the segment when replay is attempted.