Fix minor potential leak in sstable2json

patch by blerer; reviewed by slebresne for CASSANDRA-7709
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/tools/SSTableExport.java
Merge branch 'cassandra-2.1.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/tools/SSTableExport.java