[flang] Add missing check for unresolved name

[tskeith]

Summary

The name in an InputItem isn't necessarily resolved if an error occurred, so it needs to be checked.

[jdoerfert]

flang/lib/Semantics/check-io.cpp

300

For a follow up maybe:
Style: What about early exits? At level 5 this really makes a difference:

void IoChecker::Enter(const parser::InputItem &spec) {
  flags_.set(Flag::DataList);
  const parser::Variable * var{std::get_if<parser::Variable>(&spec.u)});
  if (!var)
    return;

  const parser::Name &name{GetLastName(*var)};
  if (!name.symbol)
    return;

  auto *details{name.symbol->detailsIf<ObjectEntityDetails>()}; 
  if (!details)
    return;

  // TODO: Determine if this check is needed at all, and if so, replace
  // the false subcondition with a check for a whole array.  Otherwise,
  // the check incorrectly flags array element and section references.
  if (details->IsAssumedSize() && false) {
    // This check may be superseded by C928 or C1002.
    context_.Say(name.source,
     "'%s' must not be a whole assumed size array"_err_en_US,
      name.source); // C1231
  }
}

(Notwithstanding the && false...)

flang/lib/Semantics/check-io.cpp

context_.Say(
         std::move(kind), std::move(defaultKind)); // C1229
   }
 }
 
 void IoChecker::Enter(const parser::InputItem &spec) {
   flags_.set(Flag::DataList);
   if (const parser::Variable * var{std::get_if<parser::Variable>(&spec.u)}) {
     const parser::Name &name{GetLastName(*var)};
+    if (name.symbol) {
       if (auto *details{name.symbol->detailsIf<ObjectEntityDetails>()}) {
         // TODO: Determine if this check is needed at all, and if so, replace
         // the false subcondition with a check for a whole array.  Otherwise,
         // the check incorrectly flags array element and section references.
         if (details->IsAssumedSize() && false) {
           // This check may be superseded by C928 or C1002.
           context_.Say(name.source,
               "'%s' must not be a whole assumed size array"_err_en_US,
               name.source); // C1231
         }
       }
     }
   }
+}
 
 void IoChecker::Enter(const parser::InquireSpec &spec) {
   // InquireSpec context FileNameExpr
   if (std::get_if<parser::FileNameExpr>(&spec.u)) {
     SetSpecifier(IoSpecKind::File);
   }
 }