parser: Fix detection of null bytes

Also suppress misleading extra errors.

Fixes #122.

python/tests/tstLastError.py

@@ -71,8 +71,8 @@ class TestCase(unittest.TestCase):
                         (s,len(s),"dummy.xml",None,0),
                         libxml2.treeError,
                         domain=libxml2.XML_FROM_PARSER,
-                        code=libxml2.XML_ERR_TAG_NOT_FINISHED,
-                        message='Premature end of data in tag x line 1\n',
+                        code=libxml2.XML_ERR_TAG_NAME_MISMATCH,
+                        message='Opening and ending tag mismatch: a line 2 and x\n',
                         level=libxml2.XML_ERR_FATAL,
                         file='dummy.xml',
                         line=3)