[llvm-readobj/llvm-readelf] - Report a proper warning when dumping a broken dynamic relocation.

When we have a dynamic relocation with a broken symbol's st_name,
tools report a useless error: "Invalid data was encountered while parsing the file".

After this change we report a warning + "<corrupt>" as a symbol name.

Differential revision: https://reviews.llvm.org/D66734

llvm-svn: 370330