[flang] Fix crash in name resolution

[klausler]

Summary

In an error recovery situation, the name resolution code for a
SELECT TYPE statement must check the presence of an optional
expression before calling GetType() upon it.

flang/lib/Semantics/resolve-names.cpp

 }
 
 void ConstructVisitor::Post(const parser::SelectTypeStmt &x) {
   auto &association{GetCurrentAssociation()};
   if (const std::optional<parser::Name> &name{std::get<1>(x.t)}) {
     // This isn't a name in the current scope, it is in each TypeGuardStmt
     MakePlaceholder(*name, MiscDetails::Kind::SelectTypeAssociateName);
     association.name = &*name;
-    auto exprType{association.selector.expr->GetType()};
     if (ExtractCoarrayRef(association.selector.expr)) { // C1103
       Say("Selector must not be a coindexed object"_err_en_US);
     }
+    if (association.selector.expr) {
+      auto exprType{association.selector.expr->GetType()};
       if (exprType && !exprType->IsPolymorphic()) { // C1159
         Say(association.selector.source,
             "Selector '%s' in SELECT TYPE statement must be "
             "polymorphic"_err_en_US);
       }
+    }
   } else {
     if (const Symbol *
         whole{UnwrapWholeSymbolDataRef(association.selector.expr)}) {
       ConvertToObjectEntity(const_cast<Symbol &>(*whole));
       if (!IsVariableName(*whole)) {
         Say(association.selector.source, // C901
             "Selector is not a variable"_err_en_US);
         association = {};