avoided compilation warnings on gcc 3.3.x systems

diff --git a/skeletons/BOOLEAN.c b/skeletons/BOOLEAN.c
index da38772..b0ae178 100644
--- a/skeletons/BOOLEAN.c
+++ b/skeletons/BOOLEAN.c
@@ -61,7 +61,7 @@
 
 	ASN_DEBUG("Boolean length is %d bytes", (int)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;