etcdserver: truncate WAL from correct index when forcing new cluster

When loading from a backup with a snapshot and WAL, the length of WAL entries
can be lower than the current index integer value, causing a panic when
slicing off uncommitted entries. This looks for WAL entries higher than
the current index before slicing.
Merge pull request #1782 from orls/fix-forcenew-panic

etcdserver: Fix panic when restoring from backup (-force-new-cluster)