tbf: Keep the old MS object alive in extract_tlli

Currently when a second MS object has been created for an MS, because
the TLLI was not known yet, the will be detected in
gprs_rlcmac_tbf::extract_tlli and the two objects will be merged by
update_ms. But when the dl_tbf is moved from the old to the new
(second) MS object, the old MS object can get idle and be removed
before the object are merged. This can cause LLC frame loss when the
MS object is deleted immediately after getting idle (no timeout
configured).

This commit adds a guard to keep the MS object until extract_tlli has
been executed.

Sponsored-by: On-Waves ehf
diff --git a/tests/tbf/TbfTest.cpp b/tests/tbf/TbfTest.cpp
index 3c3e91f..2af7e3a 100644
--- a/tests/tbf/TbfTest.cpp
+++ b/tests/tbf/TbfTest.cpp
@@ -900,7 +900,7 @@
 	/* OSMO_ASSERT(ms->dl_tbf() == dl_tbf); */
 
 	/* No queued packets should be lost */
-	/* OSMO_ASSERT(ms->llc_queue()->size() == 2); */
+	OSMO_ASSERT(ms->llc_queue()->size() == 2);
 
 	printf("=== end %s ===\n", __func__);
 }
@@ -961,7 +961,7 @@
 	OSMO_ASSERT(ms->dl_tbf() == dl_tbf);
 
 	/* No queued packets should be lost */
-	/* OSMO_ASSERT(ms->llc_queue()->size() == 2); */
+	OSMO_ASSERT(ms->llc_queue()->size() == 2);
 
 	printf("=== end %s ===\n", __func__);
 }