[llvm-dlltool] Don't crash if no def file is provided or it can't be opened

Differential Revision: https://reviews.llvm.org/D36780

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311068 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed