[PECOFF] Fix bug that atom size was sometimes wrong.

Because of a bug, the last atom of each section contained a garbage at the
end of its data. In most cases the garbage is harmless but it could have cause
SEGV.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@189572 91177308-0d34-0410-b5e6-96231b3b80d8