tm: t_pick_branch() cannot ingnore the pending blined UACs

Empty branches shall be considered pending branches
before their final response is set.
When all the responses were dropped in failure route and a new
blind UAC was added, t_pick_branch() did not realize that
there is still at least one pending branch, it returned an error
instead. Later on, the blind UAC was tried to be canceled causing
deadlock.

modules/tm/t_reply.c

@@ -1042,8 +1042,11 @@ int t_pick_branch(int inc_branch, int inc_code, struct cell *t, int *res_code)
 			}
 			continue;
 		}
-		/* skip 'empty branches' */
-		if (!t->uac[b].request.buffer) continue;
+		/* skip 'empty branches'
+		 * An empty branch without a final response is still considered
+		 * to be a pending, incomplete branch. */
+		if ((!t->uac[b].request.buffer) && (t->uac[b].last_received>=200))
+			continue;
 		/* there is still an unfinished UAC transaction; wait now! */
 		if ( t->uac[b].last_received<200 )
 			return -2;