Fix UAF in sdp_discovery.cc

It is possible with modifications to a client to open two connections
against the same SDP discovery database.  If this happens, it becomes
possible to reference a freed instance of the discovery database in the
second connection once the first one is closed.

To guard against this, check during discovery if a database has already
been allocated, and abort iff it has.

Also, add a null check to process_service_search_attr_rsp to guard
against unchecked calls to the SDP discovery database.

Bug: 291281168
Bug: 356201480
Flag: com.android.bluetooth.flags.btsec_check_valid_discovery_database
Test: atest bluetooth_test_gd_unit, net_test_stack_sdp
Tag: #security
Ignore-AOSP-First: Security
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:c7bc0115a7cbad342628c8428af1b8181ee1ae94)
Merged-In: I754bf8292e1e0d8e90e78fa87889284e26aa5818
Change-Id: I754bf8292e1e0d8e90e78fa87889284e26aa5818