Fix a NULL-pointer crash

It seems that the current implementation of btm_sec_encrypt_change()
does not handle the case when it is called with an invalid handle,
such as inside file btu_hcif.c :

  case HCI_SET_CONN_ENCRYPTION:
    /* Device refused to start encryption. ... */
    btm_sec_encrypt_change(BTM_INVALID_HCI_HANDLE, ...)

Bug: 22791224

Change-Id: Ide9404d0c82819399cf258ae3f90c25b352f1e20