Bluetooth: Fix to avoid NULL pointer dereferences

Bluetooth: Fix to avoid NULL pointer dereferences

This fix for avoiding NULL pointer dereferences:
 - Added check for state checking in btm_remove_acl function
   to avoid sending disconnect command if already in Disconnecting
   state.
 - Added NULL check for 'pin_code' in btif_dm_pin_reply function
 - Null check before accessing in BD interface layer configuration
   node variables.
 - Add NULL check before accessing p_bd_addr variable to avoid null
    pointer exception while referring to invalid memory.

Change-Id: Ib7ed45b6a1692785a45224d739a564f767e5b10f