sr_driver_init(): Improve checks.

sr_driver_init(): Improve checks.

Check the relevant arguments for != NULL before calling the actual
driver-specific function, so that the driver can safely assume those
arguments are non-NULL. This removes the need to duplicate these
checks in every driver.

Also, change one SR_ERR to the more correct SR_ERR_MALLOC, and assign
sr_ctx in the rigol-ds1xx2's hw_init() function, like all the other
drivers do.