Prevent premature freeing of queued GATT command

In bta_gattc_disc_cmpl, ensure that a queued command is not freed
before it is fully handled.
This prevents a potential crash caused by accessing a freed command.

Before this change, it only checked p_q_cmd to detect if
the command is queued but in bta_gattc_cfg_mtu,
the command is enqueued to p_q_cmd_queue,
and p_q_cmd remains NULL, so it failed to check
and freed the command

Bug: 315905939
Test: atest BluetoothInstrumentationTests
Test: manually connect multiple clients simultaneously
Flag: EXEMPT strict checking
Change-Id: I619e668c5d8831d857a032abeb63a159a1cb41f9