Fix crash with HTML output and empty input (Issue #560)

Author: michaelrsweet

CHANGES.md

@@ -7,6 +7,8 @@ v1.9.24 - YYYY-MM-DD
 - Updated the maximum image dimensions for RGBA images (Issue #557)
 - Added/updated memory allocation checks (Issue #558)
 - Updated error messages for consistency.
+- Fixed a crash bug when generating HTML output from empty HTML input files
+  (Issue #560)
 
 
 v1.9.23 - 2026-01-28

htmldoc/html.cxx

@@ -1010,7 +1010,8 @@ static void
 update_links(tree_t *t,			/* I - Document tree */
              uchar  *filename)		/* I - Current filename */
 {
-  tree_t	*doc = t->parent;	/* Top of document */
+  tree_t	*doc = t ? t->parent : NULL;
+					/* Top of document */
   link_t	*link;			/* Link */
   uchar		*href;			/* Reference name */
   uchar		newhref[1024];		/* New reference name */

htmldoc/htmlsep.cxx

@@ -1108,7 +1108,8 @@ static void
 update_links(tree_t *t,			// I - Document tree
              int    *heading)		// I - Current heading
 {
-  tree_t	*doc = t->parent;	// Top of document
+  tree_t	*doc = t ? t->parent : NULL;
+					// Top of document
   link_t	*link;			// Link
   uchar		*href;			// Reference name
   uchar		newhref[1024];		// New reference name