[BOLT][NFC] Fix possible use-after-free

[treapster]

Summary

If NewName twine has reference to the old name, then after Section.Name = NewName.str(); this reference is invalidated, so we cannot use NewName.str() anymore.

[rafauler]

LGTM, thanks!

bolt/lib/Core/BinaryContext.cpp

void BinaryContext::renameSection(BinarySection &Section,
                                   const Twine &NewName) {
   auto Itr = Sections.find(&Section);
   assert(Itr != Sections.end() && "Section must exist to be renamed.");
   Sections.erase(Itr);
 
   deregisterSectionName(Section);
 
   Section.Name = NewName.str();
-  Section.setOutputName(NewName);
+  Section.setOutputName(Section.Name);
 
-  NameToSection.insert(std::make_pair(NewName.str(), &Section));
+  NameToSection.insert(std::make_pair(Section.Name, &Section));
 
   // Reinsert with the new name.
   Sections.insert(&Section);
 }
 
 void BinaryContext::printSections(raw_ostream &OS) const {
   for (BinarySection *const &Section : Sections)
     OS << "BOLT-INFO: " << *Section << "\n";