[CodeGen] Check the number of operands before accessing the first one

[phosek]

Summary

After D72899, it's possible to use empty associated symbol which will
get lowered to sh_link set to 0. However, if the operand is missing
altogether, getLinkedToSymbol is going to fail. Avoid the failure
by checking the number of arguments first.

[MaskRay]

LGTM if the regression test will be added in D76802 or elsewhere

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

 }
 
 static const MCSymbolELF *getLinkedToSymbol(const GlobalObject *GO,
                                             const TargetMachine &TM) {
   MDNode *MD = GO->getMetadata(LLVMContext::MD_associated);
   if (!MD)
     return nullptr;
 
+  if (!MD->getNumOperands())
+    return nullptr;
+
   const MDOperand &Op = MD->getOperand(0);
   if (!Op.get())
     return nullptr;
 
   auto *VM = dyn_cast<ValueAsMetadata>(Op);
   if (!VM)
     report_fatal_error("MD_associated operand is not ValueAsMetadata");