Reset ParagraphBuilder after build() (#7401)

* Reset ParagraphBuilder after build()

testing/dart/paragraph_builder_test.dart

@@ -17,4 +17,11 @@ void main() {
     expect(paragraph.width, isNonZero);
     expect(paragraph.height, isNonZero);
   });
+
+  test('PushStyle should not segfault after build()', () {
+    final ParagraphBuilder paragraphBuilder =
+        ParagraphBuilder(ParagraphStyle());
+    paragraphBuilder.build();
+    paragraphBuilder.pushStyle(TextStyle());
+  });
 }

third_party/txt/src/txt/paragraph_builder.cc

@@ -84,6 +84,7 @@ std::unique_ptr<Paragraph> ParagraphBuilder::Build() {
   paragraph->SetText(std::move(text_), std::move(runs_));
   paragraph->SetParagraphStyle(paragraph_style_);
   paragraph->SetFontCollection(font_collection_);
+  SetParagraphStyle(paragraph_style_);
   return paragraph;
 }