lapd_sap_stop(): fix use of uninitialized stack memory
DL-RELEASE in lapd_core can be done in two ways: by sending DISC and waiting for UA, or a purely local clearing of state. The selection between these two modes is communicated via an extended field in struct osmo_dlsap_prim that extends beyond the standard struct osmo_prim_hdr. lapd_sap_stop() is a wrapper function that sends DL-RELEASE to lapd_core; its only current users are the vendor-specific E1 BTS handlers for Ericsson and Nokia in OsmoBSC, and both of those need local release handling. This wrapper function does allocate correctly-sized struct osmo_dlsap_prim on the stack, but until now it failed to initialize the extended field - hence the actual behavior was subject to laws of chance, depending on uninitialized content of stack memory.
