avoided compilation warnings on gcc 3.3.x systems

diff --git a/skeletons/NativeInteger.c b/skeletons/NativeInteger.c
index 759fe27..b0cc3b7 100644
--- a/skeletons/NativeInteger.c
+++ b/skeletons/NativeInteger.c
@@ -74,7 +74,7 @@
 	/*
 	 * Make sure we have this length.
 	 */
-	(char *)buf_ptr += rval.consumed;
+	buf_ptr = ((char *)buf_ptr) + rval.consumed;
 	size -= rval.consumed;
 	if(length > (ber_tlv_len_t)size) {
 		rval.code = RC_WMORE;