[Object] fixed invalid symbol handling in ELFObjectFile::getSymbolName

Found a bug in ElfObjectFile.h that occurred when there was an invalid Symbol Name in an object file. This error affected the behavior of the Expected<> value and leading it to abort, rather than behave as normal. I found this as I was adding tests to llvm-cm, as prompted by @jhenderson.

Without this fix, upon encountering an invalid symbol and trying tot l get its name, the program states that
```Expected<T> must be checked before access or destruction```
 and aborts.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D154665