[llvm-objdump] avoid crash disassembling unknown instruction

Disassembly of instructions can fail when llvm-objdump is not given the right set of
architecture features, for example when the source is compiled with:

  clang -march=..+ext1+ext2

and disassembly is attempted with:

  llvm-objdump -mattr=+ext1

This patch avoids further analysing unknown instructions (as was happening
before) when disassembly has failed.

Differential Revision: https://reviews.llvm.org/D73531