base: genlock: Check for instances where handle is NULL

Check for the possibility of NULL handles passed into the
in-kernel API functions and return error where appropriate.
There is a non-zero chance that the private_data will be
cleared while the FD is still active, so check in the ioctl()
function as well.

CRs-fixed: 332835
Change-Id: Ic0dedbada2713080fef79ca188a87c578bec6d2f
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>