mm-camera2:isp2: Handle use after free buffer

In the code, start_fetch can try to access the
buffer pointer variable after free, as the
same pointer can be freed at RELEASE_BUF call
at the same time.

Change-Id: Ic83f22336504cf67afe12131f791eee25477f011
Signed-off-by: Meera Gande <mgande@codeaurora.org>