Fix crash with dyn TS when using direct pcu

It seems there may be a race conditon where lower layers (direct PCU)
send UL blocks to us while the PDCH was already disabled (due to a call
entering on a dynamic TS).
As the PDCH is disabled, the ULC is NULL and shouldn't be used before
being enabled again.

Related: OS#5222
Change-Id: I4b8931f0cc7cfc787a1cc35196295402524b15c3
diff --git a/src/pcu_l1_if.cpp b/src/pcu_l1_if.cpp
index 5aa8849..4530e1a 100644
--- a/src/pcu_l1_if.cpp
+++ b/src/pcu_l1_if.cpp
@@ -281,6 +281,11 @@
 {
 	int rc;
 
+	if (!pdch->is_enabled()) {
+		LOGPDCH(pdch, DL1IF, LOGL_INFO, "Received DATA.ind (PDTCH) on disabled TS\n");
+		return -EINVAL;
+	}
+
 	rc = pdch->rcv_block(data, len, fn, meas);
 	pdch_ulc_expire_fn(pdch->ulc, fn);
 	return rc;