If we get an invalid packet type here the buf pointer will be NULL.
Going to the cleanup section at the end of the function would trigger
a bt_but_put() call which would cause a NULL pointer access. Directly
returning from the function is the right thing to do instead.
Change-Id: I0c18646e0820cf829ef8aa3f77835ba0a14375b5
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>