Fix handling of unexpected EOF in xmlParseContent

Readd the XML_ERR_TAG_NOT_FINISHED error on unexpected EOF which was
removed in commit 62150ed2.

This commit also introduced a regression for direct users of
xmlParseContent. Unclosed tags weren't checked.

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_LTSLASH_REQUIRED,
-                        message='EndTag: \'</\' not found\n',
+                        code=libxml2.XML_ERR_TAG_NOT_FINISHED,
+                        message='Premature end of data in tag x line 1\n',
                         level=libxml2.XML_ERR_FATAL,
                         file='dummy.xml',
                         line=3)