FakeVim: Check handler pointer before using it

Shouldn't happen in that case. But better safe than sorry.

Change-Id: I8461fec32c23e7efe6c949724cf522b58b703acc
Reviewed-by: Lukas Holecek <hluk@email.cz>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>