fixup! hooks: add custom post-command hook config

The handle_hook_replacement() call in list_hooks_add_default() was
added to interject VFS hook behavior. It dereferences `options->args`,
but `options` can be NULL when called via hook_exists() which passes
NULL for the options parameter through list_hooks().

This causes a UBSAN runtime error (null pointer member access) and
SIGABRT on every code path that calls hook_exists(), which includes
rebase, checkout, commit, and many other commands.

Guard the dereference with an `options` NULL check.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

hook.c

@@ -245,7 +245,8 @@ static void list_hooks_add_default(struct repository *r, const char *hookname,
 	struct hook *h;
 
 	/* Interject hook behavior depending on strategy. */
-	if (r && handle_hook_replacement(r, hookname, &options->args))
+	if (r && options &&
+	    handle_hook_replacement(r, hookname, &options->args))
 		return;
 
 	hook_path = find_hook(r, hookname);