Improve block bounds checking in CDEF (#436)

* Improve block bounds checking in CDEF

The block bounds calculation in CDEF was doing nothing due to a
calculation error; due to misunderstanding why, a second, moredetailed
guard was added later that should have been unneccessary.

Correct the earlier/easier check to function properly, remove the more
complex and now unneccessary later guard.

* Move CDEF strength calculaiton outside block loop

Basic CDEF strength is signaled and calculated by superblock; no need
to repeat for each block