gatt: do not dangle pointers

When enqueueing a new todo item, make sure its "next" is NULL to avoid
random traverals into neverland. This only causes issues if more than
one GATT client at once operates and thus the queue depth > 1. Otherwise
all goes well anyways since all items are by default zero-filled and
queueing is not heavy.

BUG=b:74813838
TEST=test app stable with 2 GATT clients at once
Change-Id: I408b99207e00c2a5d2b1643493c8aa35da19c662
1 file changed