Fixing a -Wsign-compare warning; NFC.

I think it might make sense to make COFF::MaxNumberOfSections16 be a uint32_t, however, that may have wider-reaching implications in other projects, which is why I did not change that declaration.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220384 91177308-0d34-0410-b5e6-96231b3b80d8