fix a NULL pointer access on deroff() failure;

could be triggered with '.SS ""';
reported by Michael <Stapelberg at debian>

1 files changed, 2 insertions, 0 deletions

diff --git a/html.c b/html.c
index 1f1d8729..e644fa21 100644
--- a/html.c
+++ b/html.c
@@ -250,6 +250,8 @@ html_make_id(const struct roff_node *n)
 
 	buf = NULL;
 	deroff(&buf, n);
+	if (buf == NULL)
+		return NULL;
 
 	/* http://www.w3.org/TR/html5/dom.html#the-id-attribute */