gprs_ns2: don't OSMO_ASSERT() while freeing NS-VC.

When freeing the NS-VC there are cases when the NSE is still alive.
This means that gprs_ns2_free_nsvc() calls  ns2_prim_status_ind(),
which in turn calls ns2_count_transfer_cap().  The latter must deal
with such a situation rather than OSMO_ASSERT()

Change-Id: I4f32dee705c9886717339fe7cce5b4cd8d2c18f7
diff --git a/src/gb/gprs_ns2.c b/src/gb/gprs_ns2.c
index a79dd05..c56b0b5 100644
--- a/src/gb/gprs_ns2.c
+++ b/src/gb/gprs_ns2.c
@@ -1455,8 +1455,9 @@
 		if (ns2_vc_is_unblocked(nsvc))
 			active_nsvcs++;
 	}
-	/* an alive nse should always have active_nsvcs */
-	OSMO_ASSERT(active_nsvcs);
+
+	if (!active_nsvcs)
+		return 0;
 
 	active_binds = talloc_zero_array(nse, struct gprs_ns2_vc_bind*, active_nsvcs);
 	if (!active_binds)