[BOLT] fix DIE traversal incorrect loop termination condition. (#208450)
The DIE traversal loop in partitionCUs() used an incorrect termination
condition, causing it to read past the end of the CU. Fix the loop to
stop at NextCUOffset so traversal no longer runs beyond the unit's
boundary.
Fixed [#208440](https://github.com/llvm/llvm-project/issues/208440).