RBS2000: Avoid segfault if ts->lapd instance doesn't exist

This happens e.g. with DAHDI driver, when the DAHDI device cannot be
opened.  Let's not prematurely seg-fault early in the RBS2000 signal
handler, but take the proper error handlign for this.

Change-Id: I9223fb1568d3db7e278f07240c4be334c6602a13

openbsc/src/libbsc/bts_ericsson_rbs2000.c

@@ -71,6 +71,8 @@ static void start_sabm_in_line(struct e1inp_line *line, int start)
 			continue;
 
 		llist_for_each_entry(link, &ts->sign.sign_links, list) {
+			if (!ts->lapd)
+				continue;
 			lapd_instance_set_profile(ts->lapd,
 						  &lapd_profile_abis_ericsson);