[PECOFF] String pointed by StringRef is not always NUL-terminated.

In order not to overrun a StringRef and copy the trailing garbage, we need to
set the maximum length to be copied by strncpy.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195688 91177308-0d34-0410-b5e6-96231b3b80d8