hw/drivers/i2s: Fix no client crash
hw/drivers/i2s: Fix no client crash

Function i2s_open() allows to pass NULL pointer for
client argument.
In one case this pointer was not checked for NULL value
before dereference.
This simply add missing NULL pointer check.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core