msm: camera: Add NULL check for stats_buf

Return error in case stat_buf is NULL in the list

CRs-Fixed: 436229
Change-Id: I42b2f61dc8e6c9e9883d0bd5d7531d62b5f0fb99
Signed-off-by: Raju P.L.S.S.S.N <rplsssn@codeaurora.org>