[gxml] Text.vala: add more error checking from spec

[Richard Hans Schwarting <rschwart src gnome org>]

commit 693f7226bb8a932424781fbcfc7d61434434e748
Author: Richard Schwarting <aquarichy gmail com>
Date:   Sat Jul 27 00:01:54 2013 -0400

    Text.vala: add more error checking from spec

 gxml/Text.vala |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/gxml/Text.vala b/gxml/Text.vala
index 82598ea..a56698f 100644
--- a/gxml/Text.vala
+++ b/gxml/Text.vala
@@ -77,6 +77,8 @@ namespace GXml {
                public Text split_text (ulong offset) {
                        Text other;
 
+                       this.check_read_only ();
+
                        /* libxml2 doesn't handle this directly, in part because it doesn't
                           allow Text siblings.  Boo! */
                        if (! this.check_index_size ("split_text", this.data.length, offset, null)) {