GATT write improvements

When doing GATT write, crash if p_clcb->p_attr_buf is null, or if
op_subtype is unknown. Both of this cases signal possible memory
corruption, and should be treated as fatal.
Properly handle GATT_CONGESTED error when doing long write.

Test: compilation test
Change-Id: Ia56f3bbd06cde2978b0ce2210ebe58042504938c