Fix possible assertion triggered in SliceFromReadCommand

patch by slebresne; reviewed by jbellis for CASSANDRA-5284
Merge branch 'cassandra-1.1' into cassandra-1.2

Conflicts:
	src/java/org/apache/cassandra/db/SliceFromReadCommand.java
Merge branch 'cassandra-1.2' into trunk
add buffer size comments
patch by Marcus Eriksson for CASSANDRA-5291
Merge branch 'cassandra-1.2' into trunk
Avoid allocating SSTableBoundedScanner during repair when the range does not intersect the sstable
patch by jbellis; reviewed by slebresne for CASSANDRA-5249
Merge branch 'cassandra-1.2' into trunk