get_dlnext(): small fix on pointer arithmetic.

src/block_header.c

@@ -134,7 +134,7 @@ void * get_dlprevious(void *ptr) {
 
 void * get_dlnext(allocator_t *allocator, void *ptr) {
     if(allocator->border_ptr != ptr) {
-        return (void *) ((char *) ptr + get_size(ptr)) + HEADER_SIZE;
+        return (void *) ((char *) ptr + get_size(ptr) + HEADER_SIZE);
     } else {
         return NULL;
     }