osd/ReplicatedPG: fix iterator corruption in cancel_copy_ops()

The cancel_copy() method removes the entry from copy_ops.  Move the
iterator forward before calling.

Fixes segfault when thrashing osds with a copy-from workload.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #663 from ceph/wip-cancel-copy

Reviewed-by: Greg Farnum <greg@inktank.com>