adding xmlMemBlocks() work on generator of an automatic API regression

* xmlmemory.c include/libxml/xmlmemory.h: adding xmlMemBlocks()
* Makefile.am gentest.py testapi.c: work on generator of an
  automatic API regression test tool.
* SAX2.c nanoftp.c parser.c parserInternals.c tree.c xmlIO.c
  xmlstring.c: various API hardeing changes as a result of running
  teh first set of automatic API regression tests.
* test/slashdot16.xml: apparently missing from CVS, commited it
Daniel

xmlstring.c

@@ -807,7 +807,6 @@ xmlCheckUTF8(const unsigned char *utf)
  *
  * Returns the storage size of
  * the first 'len' characters of ARRAY
- *
  */
 
 int
@@ -815,6 +814,9 @@ xmlUTF8Strsize(const xmlChar *utf, int len) {
     const xmlChar   *ptr=utf;
     xmlChar         ch;
 
+    if (utf == NULL)
+        return(0);
+
     if (len <= 0)
         return(0);