Fix potential NULL-dereferencing (compiler warning).

Signed-off-by: Simon Josefsson <simon@josefsson.org>

lib/structure.c

diff --git a/lib/structure.c b/lib/structure.c
index b92594e..b2be85c 100644
--- a/lib/structure.c
+++ b/lib/structure.c
@@ -418,6 +418,8 @@
     return NULL;
 
   dest_node = _asn1_add_single_node (source_node->type);
+  if (dest_node == NULL)
+    return dest_node;
 
   p_s = source_node;
   p_d = dest_node;