[aarch64][mc] Don't lookup symbols when there is no symbol lookup callback

Summary: When run under llvm-mc-disassemble-fuzzer, there is no symbol lookup callback so tryAddingSymbolicOperand() must fail gracefully instead of crashing

Reviewers: aemerson, javed.absar

Reviewed By: aemerson

Subscribers: lhames, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D51005

llvm-svn: 340287