input: qcom-hv-haptics: check playing effect pointer in IRQ handler

When uploading a constant effect after stopping FIFO play, the effect
pointer in haptics_play_info structure will be set to NULL. If there
is any pending fifo-empty IRQ gets served after that, it will trigger
a NULL pointer dereference issue. To avoid this, check the playing
effect pointer before use it in the IRQ handler.

Change-Id: Iacfea5e48d0a9519fa2c930ec821e395c43b6281
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>