[ast] Make AstPrinter print a block's labels.

Also make printing of an incomplete AST more robust.

Change-Id: I56636890deb6e38882a3f8206aff7cde3e4bab60
Reviewed-on: https://chromium-review.googlesource.com/1168498Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55009}

src/ast/prettyprinter.cc

@@ -758,8 +758,10 @@ void AstPrinter::PrintLabelsIndented(ZonePtrList<const AstRawString>* labels) {
 
 
 void AstPrinter::PrintIndentedVisit(const char* s, AstNode* node) {
+  if (node != nullptr) {
     IndentedScope indent(this, s, node->position());
     Visit(node);
+  }
 }
 
 
@@ -826,6 +828,7 @@ void AstPrinter::VisitBlock(Block* node) {
   const char* block_txt =
       node->ignore_completion_value() ? "BLOCK NOCOMPLETIONS" : "BLOCK";
   IndentedScope indent(this, block_txt, node->position());
+  PrintLabelsIndented(node->labels());
   PrintStatements(node->statements());
 }