xmlAttrSerializeTxtContent don't segfault if NULL is passed. adding an old

* tree.c: xmlAttrSerializeTxtContent don't segfault if NULL
  is passed.
* test/att7 result//att7*: adding an old regression test
  laying around on my laptop
Daniel
diff --git a/result/att7.sax b/result/att7.sax
new file mode 100644
index 0000000..c093279
--- /dev/null
+++ b/result/att7.sax
@@ -0,0 +1,24 @@
+SAX.setDocumentLocator()
+SAX.startDocument()
+SAX.internalSubset(x, , )
+SAX.elementDecl(x, 4, ...)
+SAX.elementDecl(test, 1, ...)
+SAX.attributeDecl(test, att, 1, 1, attvalue, ...)
+SAX.entityDecl(test.ent, 1, (null), (null), <test/>)
+SAX.getEntity(test.ent)
+SAX.externalSubset(x, , )
+SAX.startElement(x)
+SAX.characters(
+    , 5)
+SAX.startElement(test)
+SAX.endElement(test)
+SAX.characters(
+    , 5)
+SAX.getEntity(test.ent)
+SAX.startElement(test)
+SAX.endElement(test)
+SAX.reference(test.ent)
+SAX.characters(
+, 1)
+SAX.endElement(x)
+SAX.endDocument()