[llvm-readobj] Don't abort printing of dynamic table if string reference is invalid

If dynamic table is missing, output "dynamic strtab not found'. If the index is
out of range, output "Invalid Offset<..>".

https://bugs.llvm.org/show_bug.cgi?id=40807

Reviewed by: jhenderson, grimar, MaskRay

Differential Revision: https://reviews.llvm.org/D63084

Patch by Yuanfang Chen.

llvm-svn: 363374