msm: camera: isp: protect driver from using invalid bufq

Userspace can call some ioctls with 0 value for bufq_handle which is
currently can bypass checks in msm_isp_get_bufq and will result in
using uninitialized bufq structure, even though 0 is not a legitimate
value for bufq_handle. This change adds a check to prevent this
behaviour and to return error in case it happens.

Change-Id: I6422ec82671080cfa62fc43026b6cc33261cf11c
Signed-off-by: Petar Sivenov <psiven@codeaurora.org>