CHROMIUM: bluetooth: fix invalid pointer access to null command

This CL gives a fix that if set_event_mask() failed to prepare a
pending command and received a null pointer, it should immediately
fail as well instead of accessing the member of a invalid command.

BUG=None
TEST=emerge-kukui chromeos-kernel-4_19

Change-Id: I82740cc46e958d1b4bf4a809c2c3d7ce35acee99
Signed-off-by: Fei Shao <fshao@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1654772


Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Shyh-In Hwang <josephsih@chromium.org>
(cherry picked from commit bef0f8bc5663afa2cf547f037306d9b01bdea592)
Reviewed-on: https://chromium-review.googlesource.com/1673705