[PECOFF] Fix off-by-one error in .def file parser.

I'm fixing another bug in the parser, and I wanted to submit this
fix as a separate change as it's logically independent from the other.
I'll add a test for this shortly.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@207294 91177308-0d34-0410-b5e6-96231b3b80d8