[AST] Fix a null initializer crash for InitListExpr

[hokein]

Summary

The Initializer of a InitListExpr can be reset to null, which leads to
nullptr-acces crashes.

[hokein]

clang/lib/Sema/SemaInit.cpp

1640

I have the same question.

but if you looked at the other 2 places (same FIXME) of this file, they reset the Init only when the Result is valid. I think this place maybe an overlook.

clang/lib/Sema/SemaInit.cpp

Result =
         SemaRef.PerformCopyInitialization(Entity, expr->getBeginLoc(), expr,
                                           /*TopLevelOfInitList=*/true);
   }
 
   if (Result.isInvalid())
     hadError = true;
 
   expr = Result.getAs<Expr>();
   // FIXME: Why are we updating the syntactic init list?
-  if (!VerifyOnly)
+  if (!VerifyOnly && expr)
     IList->setInit(Index, expr);
 
   if (hadError)
     ++StructuredIndex;
   else
     UpdateStructuredListElement(StructuredList, StructuredIndex, expr);
   ++Index;
 }

clang/test/AST/ast-dump-recovery.cpp

void InitializerForAuto() {
   // drop it.
   // CHECK: `-VarDecl {{.*}} invalid unresolved_typo 'auto'
   auto unresolved_typo = gned.*[] {};
 }
 
 // Verified that the generated call operator is invalid.
 // CHECK: |-CXXMethodDecl {{.*}} invalid operator() 'auto () const -> auto'
 using Escape = decltype([] { return undef(); }());
+
+// CHECK:      VarDecl {{.*}} NoCrashOnInvalidInitList
+// CHECK-NEXT: `-RecoveryExpr {{.*}} '<dependent type>' contains-errors lvalue
+// CHECK-NEXT:   `-InitListExpr
+// CHECK-NEXT:     `-DesignatedInitExpr {{.*}} 'void'
+// CHECK-NEXT:       `-CXXNullPtrLiteralExpr {{.*}} 'nullptr_t'
+struct {
+  int& abc;
+} NoCrashOnInvalidInitList = {
+  .abc = nullptr,
+};