XSD: optional element in complex type extension

For https://bugzilla.gnome.org/show_bug.cgi?id=609796
Libxml2 fails to validate an instance document against a schema if an element
whose type is a complex extension of some base type with an optional child
element and that child element is not specified in the instance document.  For
example, suppose I have some complex type BaseType that is defined to have one
child element in a sequence group that has minOccurs set to 0

result/schemas/complex-type-extension_0_0

@@ -0,0 +1 @@
+./test/schemas/complex-type-extension_0.xml validates

test/schemas/complex-type-extension_0.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ItemsList xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+ <Item xsi:type="ItemWithNumber">
+  <Name>Name1</Name>
+  <Number>20</Number>
+ </Item>
+ <Item xsi:type="ItemWithNumber">
+  <Name>Name1</Name>
+  <Name>Name2</Name>
+  <Number>20</Number>
+ </Item>
+</ItemsList>

test/schemas/complex-type-extension_0.xsd

@@ -0,0 +1,27 @@
+<?xml version="1.0"?>
+<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
+
+ <xs:complexType name="Item" abstract="true">
+  <xs:sequence minOccurs="1" maxOccurs="unbounded">
+    <xs:element name="Name" type="xs:string"/>
+  </xs:sequence>
+ </xs:complexType>
+
+ <xs:complexType name="ItemWithNumber">
+  <xs:complexContent>
+   <xs:extension base="Item">
+    <xs:sequence>
+     <xs:element name="Number" type="xs:nonNegativeInteger"/>
+    </xs:sequence>
+   </xs:extension>
+  </xs:complexContent>
+ </xs:complexType>
+
+ <xs:element name="ItemsList">
+  <xs:complexType>
+   <xs:sequence>
+    <xs:element name="Item" type="Item" maxOccurs="unbounded"/>
+   </xs:sequence>
+  </xs:complexType>
+ </xs:element>
+</xs:schema>

xmlschemas.c

@@ -18476,8 +18476,8 @@ xmlSchemaFixupComplexType(xmlSchemaParserCtxtPtr pctxt,
 		    particle->children->children =
 			(xmlSchemaTreeItemPtr) xmlSchemaAddParticle(pctxt,
 			type->node,
-			((xmlSchemaParticlePtr) type->subtypes)->minOccurs,
-			((xmlSchemaParticlePtr) type->subtypes)->maxOccurs);
+			((xmlSchemaParticlePtr) baseType->subtypes)->minOccurs,
+			((xmlSchemaParticlePtr) baseType->subtypes)->maxOccurs);
 		    if (particle->children->children == NULL)
 			goto exit_failure;
 		    particle = (xmlSchemaParticlePtr)