lbutils/file ffree_and_set_null: err if null

free can take a null, that's fine, but my pointer
to the pointer being freed should not be null. that
is a bug.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/libreboot-utils/lib/file.c

@@ -578,7 +578,11 @@ try_err(int loop_err, int errval)
 void
 free_and_set_null(char **buf)
 {
-	if (buf == NULL || *buf == NULL)
+	if (buf == NULL)
+		err_exit(EFAULT,
+		    "null ptr (to ptr for freeing) in free_and_set_null");
+
+	if (*buf == NULL)
 		return;
 
 	free(*buf);