xmlschemastypes: ensure thread safe global initialization

Relying on a plain integer flag, with no synchronization primitives does
not give thread-safe initialization. All reads & writes of the
xmlSchemaTypesInitialized flag need to be protected by a mutex to ensure
suitable memory barriers & thus correct ordering wrt any speculative
execution.

A separate internal initializer tied to xmlParserInit is used to create
the mutex used for synchronization, similarly to how catalog.c works.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

include/private/threads.h

@@ -58,4 +58,12 @@ xmlInitRMutex(xmlRMutex *mutex);
 XML_HIDDEN void
 xmlCleanupRMutex(xmlRMutex *mutex);
 
+#ifdef LIBXML_SCHEMAS_ENABLED
+XML_HIDDEN void
+xmlInitSchemasTypesInternal(void);
+XML_HIDDEN void
+xmlCleanupSchemasTypesInternal(void);
+#endif
+
+
 #endif /* XML_THREADS_H_PRIVATE__ */