msm: ipc: Check pointer validity before dereferencing it

Transport OPEN and CLOSE events are handled by dynamically allocating a
work. Check that the work item is properly allocated before dereferencing
it.

Change-Id: Icb73548f0747494791f14f9c8014f8effd4b844c
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>