tbf_fsm: rename state NULL -> NEW

This helps distinguishing the case where a TBF is in the initial state
and the unexpected case where osmo_fsm_inst_state_name reports "NULL"
due to fi pointer being NULL.

Change-Id: Ieaabfc9fa0dedb299bcf4541783cf80e366a88c3
diff --git a/src/tbf_fsm.h b/src/tbf_fsm.h
index d62e091..614c319 100644
--- a/src/tbf_fsm.h
+++ b/src/tbf_fsm.h
@@ -43,7 +43,7 @@
 };
 
 enum tbf_fsm_states {
-	TBF_ST_NULL = 0,	/* new created TBF */
+	TBF_ST_NEW = 0,	/* new created TBF */
 	TBF_ST_ASSIGN,	/* wait for downlink assignment */
 	TBF_ST_FLOW,	/* RLC/MAC flow, resource needed */
 	TBF_ST_FINISHED,	/* flow finished, wait for release */