Adds null check before freeing LE services list

Adds null check before freeing LE services list

Prevents the crash which happens when device is
disconnected during service discovery and the
services list is not yet properly allocated
memory and populated. This change frees up the
services list after discovery complete only when
it is not null.

Change-Id: I0ced14d2ee133a332fa2fd7c456267438813a600