[llvm-readobj/elf] - Fix a crash when dumping a dynamic relocation that refer...

[llvm-readobj/elf] - Fix a crash when dumping a dynamic relocation that refer to a symbol past the EOF.

There is a possible scenario when we crash when dumping dynamic relocations.
For that we should have no section headers (to take the number of synamic symbols from)
and a dynamic relocation that refers to a symbol with an index that is too large to be in a file.

The patch fixes it.

Differential revision: https://reviews.llvm.org/D90214