add a missing check in xmlAddSibling, patch by Kris Breuker avoid

* tree.c: add a missing check in xmlAddSibling, patch by Kris Breuker
* xmlIO.c: avoid xmlAllocOutputBuffer using XML_BUFFER_EXACT which
  leads to performances problems especially on Windows.
daniel

svn path=/trunk/; revision=3820
diff --git a/xmlIO.c b/xmlIO.c
index d4dc364..855bf41 100644
--- a/xmlIO.c
+++ b/xmlIO.c
@@ -2278,6 +2278,10 @@
 	return(NULL);
     }
 
+    /* try to avoid a performance problem with Windows realloc() */
+    if (ret->buffer->alloc == XML_BUFFER_ALLOC_EXACT)
+        ret->buffer->alloc = XML_BUFFER_ALLOC_DOUBLEIT;
+
     ret->encoder = encoder;
     if (encoder != NULL) {
         ret->conv = xmlBufferCreateSize(4000);