tbf: Add GprsMs* argument to update() and use it in reuse_tbf

Since set_ms() is caled on the new DL TBF, the old DL TBF loses the
reference to the MS object. This will lead to a segfault, when
update() is called in reuse_tbf().

This commit adds an optional GprsMs* parameter to update() and uses it
for the slot allocation.

This fixes a TbfTest crash that would otherwise occur after applying
the next commit.

Sponsored-by: On-Waves ehf
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index c7a04bf..771b35b 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -869,7 +869,7 @@
 	state_flags &= GPRS_RLCMAC_FLAG_TO_MASK;
 	state_flags &= ~(1 << GPRS_RLCMAC_FLAG_CCCH);
 
-	update();
+	update(new_tbf->ms());
 
 	LOGP(DRLCMAC, LOGL_DEBUG, "%s Trigger dowlink assignment on PACCH, "
 		"because another LLC PDU has arrived in between\n",