msm: smem_log: Handle uninitialized logs

SMEM logs such as the power logs are special purpose and are only used
on some targets.  If they are not used, their associated SMEM items will
not be allocated and we therefore need access guards to prevent
accessing uninitialized pointers.

CRs-Fixed: 347770
Change-Id: I8ad2be73e76167684da827be1e4ca9121f920314
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>