error: Make xmlGetLastError return a const error

This is a slight break of the API, but users really shouldn't modify the
global error struct. The goal is to make xmlLastError use static buffers
for its strings eventually. This should warn people if they're abusing
the struct.

python/types.c

@@ -8,6 +8,7 @@
  */
 #include "libxml_wrap.h"
 #include <libxml/xpathInternals.h>
+#include <string.h>
 
 #if PY_MAJOR_VERSION >= 3
 #define PY_IMPORT_STRING_SIZE PyUnicode_FromStringAndSize
@@ -963,15 +964,30 @@ libxml_xmlSchemaValidCtxtPtrWrap(xmlSchemaValidCtxtPtr valid)
 }
 #endif /* LIBXML_SCHEMAS_ENABLED */
 
+static void
+libxml_xmlDestructError(PyObject *cap) {
+    xmlErrorPtr err = (xmlErrorPtr) PyCapsule_GetPointer(cap, "xmlErrorPtr");
+    xmlResetError(err);
+    xmlFree(err);
+}
+
 PyObject *
-libxml_xmlErrorPtrWrap(xmlErrorPtr error)
+libxml_xmlErrorPtrWrap(const xmlError *error)
 {
     PyObject *ret;
+    xmlErrorPtr copy;
 
     if (error == NULL) {
         Py_INCREF(Py_None);
         return (Py_None);
     }
-    ret = PyCapsule_New((void *) error, (char *) "xmlErrorPtr", NULL);
+    copy = xmlMalloc(sizeof(*copy));
+    if (copy == NULL) {
+        Py_INCREF(Py_None);
+        return (Py_None);
+    }
+    memset(copy, 0, sizeof(*copy));
+    xmlCopyError(error, copy);
+    ret = PyCapsule_New(copy, "xmlErrorPtr", libxml_xmlDestructError);
     return (ret);
 }