Check validity of GATT client ID before unregistering

GATT would always de-register HOGP host if de-registration was requested
for unknown GATT client ID. This used to occasionally corrupt
(b/266474582) the HOGP host state which was fixed in
Iaff8495bbfcc1288b7011380474fa6a0eac302cc.
During BT shutdown, the GATT client cleanup may complete before
de-register requests from all clients are serviced. In such case,
handling of pending de-register requests results in null pointer access.

Bug: 277181689
Test: Regression
Change-Id: I6a56e0561212e9361b960b174127c95fe4b686fd