[Object] Prevent null pointer dereference in BigArchiveMemberHeader

[vhscampos]

Summary

The pointer argument Err from BigArchiveMemberHeader's constructor can
be null. This pointer was dereferenced without any null checks.

This patch adds a check to avoid a null dereference.

[vhscampos]

For reference, a null pointer can reach this code from Archive.cpp line 458.

llvm/lib/Object/Archive.cpp

BigArchiveMemberHeader::BigArchiveMemberHeader(const Archive *Parent,
                                                const char *RawHeaderPtr,
                                                uint64_t Size, Error *Err)
     : CommonArchiveMemberHeader<BigArMemHdrType>(
           Parent, reinterpret_cast<const BigArMemHdrType *>(RawHeaderPtr)) {
   if (RawHeaderPtr == nullptr)
     return;
   ErrorAsOutParameter ErrAsOutParam(Err);
 
-  if (Size < getSizeOf())
-    *Err = createMemberHeaderParseError(this, RawHeaderPtr, Size);
+  if (Size < getSizeOf()) {
+    Error SubErr = createMemberHeaderParseError(this, RawHeaderPtr, Size);
+    if (Err)
+      *Err = std::move(SubErr);
+  }
 }
 
 // This gets the raw name from the ArMemHdr->Name field and checks that it is
 // valid for the kind of archive.  If it is not valid it returns an Error.
 Expected<StringRef> ArchiveMemberHeader::getRawName() const {
   char EndCond;
   auto Kind = Parent->kind();
   if (Kind == Archive::K_BSD || Kind == Archive::K_DARWIN64) {