Chasing dead assignments reported by clang-scan

* SAX2.c dict.c error.c hash.c nanohttp.c parser.c python/libxml.c
  relaxng.c runtest.c tree.c valid.c xinclude.c xmlregexp.c xmlsave.c
  xmlschemas.c xpath.c xpointer.c: mostly removing unneded affectations,
  but this led to a few real bugs and some part not yet understood
  (relaxng/interleave)

python/libxml.c

@@ -2416,6 +2416,7 @@ libxml_prev(ATTRIBUTE_UNUSED PyObject * self, PyObject * args)
 
                 res = (xmlNodePtr) attr->prev;
             }
+            break;
         case XML_NAMESPACE_DECL:
             res = NULL;
             break;