fixed a serious problem when substituing entities using the Reader, the

* parser.c xmlreader.c include/libxml/parser.h: fixed a serious
  problem when substituing entities using the Reader, the entities
  content might be freed and if rereferenced would crash
* Makefile.am test/* result/*: added a new test case and a new
  test operation for the reader with substitution of entities.
Daniel
diff --git a/result/dtd3.rde b/result/dtd3.rde
new file mode 100644
index 0000000..20cbf2a
--- /dev/null
+++ b/result/dtd3.rde
@@ -0,0 +1,4 @@
+0 10 doc 0 0
+0 1 doc 0 0
+1 3 #text 0 1 This is a valid document !
+0 15 doc 0 0