LLD: Hexagon: Fix dereferencing end() iterator.

[ruiu]

Summary

findAbsoluteAtom() returns absoluteAtom().end() if no atom is found.
Dereferencing end() value results an undefined behavior.

[shankarke]

LGTM.

[Eugene.Zelenko]

Committed in r233765.

lib/ReaderWriter/ELF/Hexagon/HexagonTargetHandler.h

 template <class HexagonELFType>
 class HexagonTargetLayout final : public TargetLayout<HexagonELFType> {
 public:
   enum HexagonSectionOrder {
     ORDER_SDATA = 205
   };
 
   HexagonTargetLayout(HexagonLinkingContext &hti)
-      : TargetLayout<HexagonELFType>(hti), _sdataSection(nullptr),
-        _gotSymAtom(nullptr), _cachedGotSymAtom(false) {
+      : TargetLayout<HexagonELFType>(hti), _sdataSection() {
     _sdataSection = new (_alloc) SDataSection<HexagonELFType>(hti);
   }
 
   /// \brief Return the section order for a input section
   Layout::SectionOrder getSectionOrder(
       StringRef name, int32_t contentType, int32_t contentPermissions) override {
     if ((contentType == DefinedAtom::typeDataFast) ||
        (contentType == DefinedAtom::typeZeroFillFast))
getSegmentType(Section<HexagonELFType> *section) const override {
     return DefaultLayout<HexagonELFType>::getSegmentType(section);
   }
 
   Section<HexagonELFType> *getSDataSection() const {
     return _sdataSection;
   }
 
   uint64_t getGOTSymAddr() {
-    if (!_cachedGotSymAtom) {
-      auto gotAtomIter = this->findAbsoluteAtom("_GLOBAL_OFFSET_TABLE_");
-      _gotSymAtom = (*gotAtomIter);
-      _cachedGotSymAtom = true;
+    if (!_gotSymAtom.hasValue()) {
+      auto iter = this->findAbsoluteAtom("_GLOBAL_OFFSET_TABLE_");
+      _gotSymAtom = (iter == this->absoluteAtoms().end()) ? nullptr : *iter;
     }
-    if (_gotSymAtom)
-      return _gotSymAtom->_virtualAddr;
+    if (*_gotSymAtom)
+      return (*_gotSymAtom)->_virtualAddr;
     return 0;
   }
 
 private:
   llvm::BumpPtrAllocator _alloc;
-  SDataSection<HexagonELFType> *_sdataSection;
-  AtomLayout *_gotSymAtom;
-  bool _cachedGotSymAtom;
+  SDataSection<HexagonELFType> *_sdataSection = nullptr;
+  llvm::Optional<AtomLayout *> _gotSymAtom;
 };
 
 /// \brief TargetHandler for Hexagon
 class HexagonTargetHandler final :
     public DefaultTargetHandler<HexagonELFType> {
 public:
   HexagonTargetHandler(HexagonLinkingContext &targetInfo);