diff -up shadow-4.2.1/lib/groupio.c.merge-group shadow-4.2.1/lib/groupio.c
--- shadow-4.2.1/lib/groupio.c.merge-group	2014-11-26 14:33:54.039581662 +0100
+++ shadow-4.2.1/lib/groupio.c	2014-11-26 14:46:02.841852886 +0100
@@ -335,8 +335,7 @@ static /*@null@*/struct commonio_entry *
 		errno = ENOMEM;
 		return NULL;
 	}
-	snprintf(new_line, new_line_len, "%s\n%s", gr1->line, gr2->line);
-	new_line[new_line_len] = '\0';
+	snprintf(new_line, new_line_len + 1, "%s\n%s", gr1->line, gr2->line);
 
 	/* Concatenate the 2 list of members */
 	for (i=0; NULL != gptr1->gr_mem[i]; i++);