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.h b/src/tbf.h
index 840125a..82ade4e 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -131,7 +131,7 @@
 
 	int rlcmac_diag();
 
-	int update();
+	int update(GprsMs *ms = NULL);
 	void handle_timeout();
 	void stop_timer();
 	void stop_t3191();