block: prevent access to NULL pointer in req->part

Block layer is accessing req->part without checking for NULL
pointer access, it is done when statistcs is not fully
initialized. Preventing the NULL pointer access effects only
the statistics update.

Change-Id: I45c91c074ecec1c3849f4f36185edcc6db35383c
Signed-off-by: Yaniv Gardi <ygardi@codeaurora.org>
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>