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/dtd5.rde b/result/dtd5.rde
new file mode 100644
index 0000000..66773a3
--- /dev/null
+++ b/result/dtd5.rde
@@ -0,0 +1,11 @@
+0 10 doc 0 0
+0 1 doc 0 0
+1 1 a 0 0
+2 3 #text 0 1 This
+1 15 a 0 0
+1 3 #text 0 1  is a 
+1 1 b 0 0
+2 3 #text 0 1 valid
+1 15 b 0 0
+1 3 #text 0 1  document
+0 15 doc 0 0