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

[dsanders]

Summary

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

[dsanders]

This seems like an obvious fix but I don't know whether it was deliberately or accidentally mandatory.

@aemerson: Are you aware of any reasons it was mandatory?

[aemerson]

Looks like Lang added the original code, but LGTM anyway. I even think we did this exact change downstream a few years ago at ARM...

[dsanders]

Thanks

llvm/trunk/lib/Target/AArch64/Disassembler/AArch64ExternalSymbolizer.cpp

 /// returns zero and isBranch is Fail then the Opcode of the MCInst is
 /// tested and for ADRP an other instructions that help to load of pointers
 /// a symbol look up is done to see it is returns a specific reference type
 /// to add to the comment stream.  This function returns Success if it adds
 /// an operand to the MCInst and Fail otherwise.
 bool AArch64ExternalSymbolizer::tryAddingSymbolicOperand(
     MCInst &MI, raw_ostream &CommentStream, int64_t Value, uint64_t Address,
     bool IsBranch, uint64_t Offset, uint64_t InstSize) {
+  if (!SymbolLookUp)
+    return false;
   // FIXME: This method shares a lot of code with
   //        MCExternalSymbolizer::tryAddingSymbolicOperand. It may be possible
   //        refactor the MCExternalSymbolizer interface to allow more of this
   //        implementation to be shared.
   //
   struct LLVMOpInfo1 SymbolicOp;
   memset(&SymbolicOp, '\0', sizeof(struct LLVMOpInfo1));
   SymbolicOp.Value = Value;