mds: be more careful about decoding LogEvents

We need to wrap the full decode section or we can abort the process
if there's an issue (which we may want to just skip by).

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 73289b34b0be5b6612e38944794d59b5e789f841)