obj2yaml: Don't crash if the characteristics field is zero

obj2yaml would fail when seeing a Weak External auxiliary record with a
characteristics field holding zero instead of one of
IMAGE_WEAK_EXTERN_SEARCH_NOLIBRARY, IMAGE_WEAK_EXTERN_SEARCH_NOLIBRARY,
or IMAGE_WEAK_EXTERN_SEARCH_NOLIBRARY.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205911 91177308-0d34-0410-b5e6-96231b3b80d8