[flang] Fix crash on USE error

[klausler]

Summary

Handle the case of a non-generic procedure that is USE associated
into a scope that has a generic interface of the same name with an
appropriate error rather than crashing.

flang/lib/Semantics/resolve-names.cpp

ModuleVisitor::SymbolRename ModuleVisitor::AddUse(
   }
   auto &localSymbol{MakeSymbol(localName)};
   DoAddUse(useName, localName, localSymbol, *useSymbol);
   return {&localSymbol, useSymbol};
 }
 
 // symbol must be either a Use or a Generic formed by merging two uses.
 // Convert it to a UseError with this additional location.
-static void ConvertToUseError(
+static bool ConvertToUseError(
     Symbol &symbol, const SourceName &location, const Scope &module) {
   const auto *useDetails{symbol.detailsIf<UseDetails>()};
   if (!useDetails) {
-    auto &genericDetails{symbol.get<GenericDetails>()};
-    useDetails = &genericDetails.uses().at(0)->get<UseDetails>();
+    if (auto *genericDetails{symbol.detailsIf<GenericDetails>()}) {
+      if (!genericDetails->uses().empty()) {
+        useDetails = &genericDetails->uses().at(0)->get<UseDetails>();
       }
+    }
+  }
+  if (useDetails) {
     symbol.set_details(
         UseErrorDetails{*useDetails}.add_occurrence(location, module));
+    return true;
+  } else {
+    return false;
+  }
 }
 
 // If a symbol has previously been USE-associated and did not appear in a USE
 // ONLY clause, erase it from the current scope.  This is needed when a name
 // appears in a USE rename clause.
 void ModuleVisitor::EraseRenamedSymbol(const Symbol &useSymbol) {
   const SourceName &name{useSymbol.name()};
   if (const Symbol * symbol{FindInScope(name)}) {
if (localUltimate.has<DerivedTypeDetails>()) {
       Symbol &newSymbol{MakeSymbol(localName,
           useUltimate.attrs() & ~Attrs{Attr::PUBLIC, Attr::PRIVATE},
           UseDetails{localName, useUltimate})};
       newSymbol.flags() = useSymbol.flags();
       return;
     }
   }
   if (!combine) {
-    if (localSymbol.has<UseDetails>() || localSymbol.has<GenericDetails>()) {
-      ConvertToUseError(localSymbol, location, *useModuleScope_);
-    } else {
+    if (!ConvertToUseError(localSymbol, location, *useModuleScope_)) {
       Say(location,
           "Cannot use-associate '%s'; it is already declared in this scope"_err_en_US,
           localName)
           .Attach(localSymbol.name(), "Previous declaration of '%s'"_en_US,
               localName);
     }
     return;
   }