[llvm-readobj] - Don't crash when dumping .stack_sizes and unable to find a relocation resolver.

The crash might happen when we have either a broken or unsupported object
and trying to resolve relocations when dumping the .stack_sizes section.

For the test case I used a 32-bits ELF header and a 64-bit relocation.
In this case a null pointer is returned by the code instead of the relocation
resolver function and then we crash.

Differential revision: https://reviews.llvm.org/D67962

llvm-svn: 372838