Don't allow invalid node names in XML::Node#name=

spec/std/xml/xml_spec.cr

@@ -257,6 +257,31 @@ describe XML do
     root.name.should eq("last-name")
   end
 
+  it "doesn't set invalid node name" do
+    doc = XML.parse(<<-XML
+      <?xml version='1.0' encoding='UTF-8'?>
+      <name>John</name>
+      XML
+    )
+    root = doc.root.not_nil!
+
+    expect_raises(XML::Error, "Invalid node name") do
+      root.name = " foo bar"
+    end
+
+    expect_raises(XML::Error, "Invalid node name") do
+      root.name = "foo bar"
+    end
+
+    expect_raises(XML::Error, "Invalid node name") do
+      root.name = "1foo"
+    end
+
+    expect_raises(XML::Error, "Invalid node name") do
+      root.name = "\0foo"
+    end
+  end
+
   it "gets encoding" do
     doc = XML.parse(<<-XML
         <?xml version='1.0' encoding='UTF-8'?>

src/xml/libxml2.cr

@@ -294,6 +294,8 @@ lib LibXML
   fun xmlGetNsList(doc : Doc*, node : Node*) : NS**
 
   fun xmlSetProp(node : Node*, name : UInt8*, value : UInt8*) : Attr*
+
+  fun xmlValidateNameValue(value : UInt8*) : Int
 end
 
 LibXML.xmlGcMemSetup(

src/xml/node.cr

@@ -274,7 +274,18 @@ struct XML::Node
     if document? || text? || cdata? || fragment?
       raise XML::Error.new("Can't set name of XML #{type}", 0)
     end
-    LibXML.xmlNodeSetName(self, name.to_s)
+
+    name = name.to_s
+
+    if name.includes? '\0'
+      raise XML::Error.new("Invalid node name: #{name.inspect} (contains null character)")
+    end
+
+    if LibXML.xmlValidateNameValue(name) == 0
+      raise XML::Error.new("Invalid node name: #{name.inspect}", 0)
+    end
+
+    LibXML.xmlNodeSetName(self, name)
   end
 
   # Returns the namespace for this node or `nil` if not found.