[clang-doc] Add check for pointer validity

[paulkirth]

Summary

clang-doc would SEGV when running over the Fuchsia code base.
This patch adds a check to avoid dereferencing potentially null pointers
in the Values vector. These pointers were either never valid or had been
invalidated when the underlying pointer in std::unique_ptr was moved from,
hence making it nullptr within the vector.

[phosek]

LGTM

clang-tools-extra/clang-doc/Representation.cpp

 
 namespace {
 
 const SymbolID EmptySID = SymbolID();
 
 template <typename T>
 llvm::Expected<std::unique_ptr<Info>>
 reduce(std::vector<std::unique_ptr<Info>> &Values) {
-  if (Values.empty())
+  if (Values.empty() || !Values[0])
     return llvm::createStringError(llvm::inconvertibleErrorCode(),
                                    "no value to reduce");
   std::unique_ptr<Info> Merged = std::make_unique<T>(Values[0]->USR);
   T *Tmp = static_cast<T *>(Merged.get());
   for (auto &I : Values)
     Tmp->merge(std::move(*static_cast<T *>(I.get())));
   return std::move(Merged);
 }
void reduceChildren(std::vector<EnumInfo> &Children,
   }
 }
 
 } // namespace
 
 // Dispatch function.
 llvm::Expected<std::unique_ptr<Info>>
 mergeInfos(std::vector<std::unique_ptr<Info>> &Values) {
-  if (Values.empty())
+  if (Values.empty() || !Values[0])
     return llvm::createStringError(llvm::inconvertibleErrorCode(),
                                    "no info values to merge");
 
   switch (Values[0]->IT) {
   case InfoType::IT_namespace:
     return reduce<NamespaceInfo>(Values);
   case InfoType::IT_record:
     return reduce<RecordInfo>(Values);