deprecate the non-boundchecking Sprintf functions, add Snprintf this

* include/libxml/valid.h debugXML.c valid.c: deprecate
  the non-boundchecking Sprintf functions, add Snprintf
  this should close bug #57984
Daniel
diff --git a/debugXML.c b/debugXML.c
index ddf9657..83a4d9d 100644
--- a/debugXML.c
+++ b/debugXML.c
@@ -264,7 +264,7 @@
 	char buf[5001];
 
 	buf[0] = 0;
-	xmlSprintfElementContent(buf, elem->content, 1);
+	xmlSnprintfElementContent(buf, 5000, elem->content, 1);
 	buf[5000] = 0;
 	fprintf(output, "%s", buf);
     }