Do not send write requestif tcb is closed

If tcb is closed, the mtu resets to 0.
Although I couldn't find a exact path but
gatt_send_write can be invoked after a tcb is closed.

This change prevents this scenario by rejecting
the request.

Flag: strict bound check
Bug: 365910287
Test: atest GattClientTest

Change-Id: I1943dff545cba51bdc50a2d22dd335c1bdb5eaa6