Fixed a crash while copying comments to properties.

Signed-off-by: Dimitar Dobrev <dpldobrev@yahoo.com>

src/Generator/Passes/GetterSetterToPropertyAdvancedPass.cs

@@ -159,6 +159,8 @@ namespace CppSharp.Passes
                         comment.Kind = getter.Comment.Kind;
                         comment.BriefText = getter.Comment.BriefText;
                         comment.Text = getter.Comment.Text;
+                        if (getter.Comment.FullComment != null)
+                        {
                             comment.FullComment = new FullComment();
                             comment.FullComment.Blocks.AddRange(getter.Comment.FullComment.Blocks);
                             if (setter != null && setter.Comment != null)
@@ -167,6 +169,7 @@ namespace CppSharp.Passes
                                 comment.Text += Environment.NewLine + setter.Comment.Text;
                                 comment.FullComment.Blocks.AddRange(setter.Comment.FullComment.Blocks);
                             }
+                        }
                         property.Comment = comment;
                     }
                     type.Properties.Add(property);