msm: camera: isp: check for valid buffer

In case of recovery there can be no available buffer so we need to
check for that condition or a null pointer dereference can happen.
This change adds error handling for that case.

Change-Id: I1b4c261a2df2ec65b6c1542deb2c286e3fb74d94
Signed-off-by: Petar Sivenov <psiven@codeaurora.org>