[LLD] Fixed crash in type merging

[aganea]

Summary

This fixes a crash when a second pass is required for the Codeview Type merging *and* the index points outside of the table (which should lead to an error being printed).

This occurs currently until MS precompiled headers .obj is added (see D45213)

[aganea]

Added test in r338423

llvm/trunk/lib/DebugInfo/CodeView/TypeStreamMerger.cpp

bool TypeStreamMerger::remapIndexFallback(TypeIndex &Idx,
                                           ArrayRef<TypeIndex> Map) {
   size_t MapPos = slotForIndex(Idx);
 
   // If this is the second pass and this index isn't in the map, then it points
   // outside the current type stream, and this is a corrupt record.
   if (IsSecondPass && MapPos >= Map.size()) {
     // FIXME: Print a more useful error. We can give the current record and the
     // index that we think its pointing to.
+    if (LastError)
       LastError = joinErrors(std::move(*LastError), errorCorruptRecord());
+    else
+      LastError = errorCorruptRecord();
   }
 
   ++NumBadIndices;
 
   // This type index is invalid. Remap this to "not translated by cvpack",
   // and return failure.
   Idx = Untranslated;
   return false;