iolog: fix reported defect from coverity scan

iolog: fix reported defect from coverity scan

Fix the two Null pointer dereferences issue reported by Coverity scan

Null pointer dereferences  (FORWARD_NULL)
Dereferencing null pointer "l->td"

Null pointer dereferences  (REVERSE_INULL)
Null-checking "p->td" suggests that it may be null, but it has already
been dereferenced on all paths leading to the check.

For aggregate read, write and trim bandwidth log, the setup_log function
gets called with NULL pointer reference for thread data. Thus before
dereferencing further we should check "l->td".

Signed-off-by: Ankit Kumar <ankit.kumar@samsung.com>