sstables: Fix estimation of pending tasks for leveled strategy
sstables: Fix estimation of pending tasks for leveled strategy

There were two underflow bugs.

1) in variable i, causing get_level() to see an invalid level and
throw an exception as a result.
2) when estimating number of pending tasks for a level.

Fixes #1603.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>
Message-Id: <cce993863d9de4d1f49b3aabe981c475700595fc.1471636164.git.1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

avikivity/scylla,scylladb/scylla,avikivity/scylla,raphaelsc/scylla,raphaelsc/scylla,duarten/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla