Fix NPE when discovering invalid GATT datatbase

When doing a discovery of invalid GATT database, where two services are
added with same start handle, characteristics are added to the first
one, where descriptors are added to the later. This cause NPE when
adding descriptor.

Bug: 34280184
Test: manual test
Change-Id: I27619098df33a641cb089b82cf4d4ffd3c6e6aea