Fix NPE in FileCacheService.sizeInBytes

patch by tjake; reviewed by jbellis for (CASSANDRA-7756)
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/io/util/RandomAccessReader.java