Fix crash during derigister GATT server

When deregister a gatt server, GATT_deregister
will use a loop to stop service one by one and
call std::list::erase in GATTS_StopService to
remove service info. But erase makes iterator lose
efficacy. If the iterator is operated after that,
Bluetooth will crash.

Add the iterator before erase.

Test: manual
Change-Id: I10f9351a95ab4922553d8a77663a0212407607aa