[Object] Always initialized StartOfFile in Archive::Child::Child constructor.

[jacek]

Summary

Fixes uninitialized memory access revealed by https://reviews.llvm.org/D146534.

In empty archives, we use Child(nullptr,nullptr,nullptr) in Archive constructor in setFirstRegular. This copies unitialized StartOfFile to FirstRegularStartOfFile, which child_begin may use later.

[efriedma]

LGTM

[thurston]

Thank you Jacek for writing this patch! I took the liberty of submitting it.

llvm/lib/Object/Archive.cpp

Archive::Child::Child(const Archive *Parent, StringRef Data,
     : Parent(Parent), Data(Data), StartOfFile(StartOfFile) {
   Header = Parent->createArchiveMemberHeader(Data.data(), Data.size(), nullptr);
 }
 
 Archive::Child::Child(const Archive *Parent, const char *Start, Error *Err)
     : Parent(Parent) {
   if (!Start) {
     Header = nullptr;
+    StartOfFile = -1;
     return;
   }
 
   Header = Parent->createArchiveMemberHeader(
       Start,
       Parent ? Parent->getData().size() - (Start - Parent->getData().data())
              : 0,
       Err);