input: misc: qti-haptics: check NULL pointer before using it

There might be a rare case that previous play has been terminated and the
next play is generating in VMAX mode so that the play effect pointer would
be set to NULL before the play IRQ being served. So check the effect
pointer before using it in the play IRQ handler.

Change-Id: I2d9ace5d09ffde885b4267e1e78771a169c57bec
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>