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.

(cherry picked from commit a73ed097420c53451e843ae61d9aadc8fefabc94)

Change-Id: If481b98a6a2ab01ab788c23bffe9df7ab8bc6aee
CRs-fixed: 332835
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>