mobile: Speculative crash fix of the SI pointer

The SIs are kept per ARFCN and for the current cell the
cs->si alias will be assigned[1]. On mobile_exit all SIs
will be freed but the alias will not be set to NULL.

This is a speculative fix but it doesn't seem to make
things worse.

Related: OS#2690

[1] cs->si = cs->list[cs->arfci].sysinfo;

Change-Id: Icf20f9aa03dd26d4bee78772b7f3da034bb34b99

src/host/layer23/src/mobile/gsm322.c

@@ -5141,6 +5141,7 @@ int gsm322_exit(struct osmocom_ms *ms)
 		}
 		cs->list[i].flags = 0;
 	}
+	cs->si = NULL;
 
 	/* store BA list */
 	ba_filename = talloc_asprintf(ms, "%s/%s.ba", config_dir, ms->name);