bta_gattc: Fix initialization and free p_q_cmd

This will prevent crash when ACL is disconnected while GATT search

Bug: 253200147
Test: atest BluetoothInstrumentationTests
Test: manual disconnect while bonding and discovering
Tag: #feature
Merged-In: If7ad51378b650b67e551f8bbec7b38d0d9c14cbd
Change-Id: If7ad51378b650b67e551f8bbec7b38d0d9c14cbd
(cherry picked from commit e8bf02dd)