gsi: Fix NULL pointer dereference on event-less channels

If gsi_start_channel() or gsi_stop_channel() are called
on a channel with no event ring allocated, there will be
null pointer dereference error.
Adding null pointer check.

Change-Id: I200aa09f49c2394041f66c33c458f7daa9aeaf8c
Signed-off-by: Ilia Lin <ilial@codeaurora.org>