llvm-readobj: Do not print NULL StringRefs

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180005 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-readobj: Check for null section pointer

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180006 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-readobj: Dump more COFF auxiliary records

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180007 91177308-0d34-0410-b5e6-96231b3b80d8