sba: fix possible memleak in SBAController::alloc()

Change-Id: I417eda155cd5b1e46dd0b05db3f507abd79121d1
diff --git a/src/sba.cpp b/src/sba.cpp
index 567babe..32735e8 100644
--- a/src/sba.cpp
+++ b/src/sba.cpp
@@ -55,13 +55,13 @@
 	int8_t trx, ts;
 	uint32_t fn;
 
+	if (!gsm48_ta_is_valid(ta))
+		return -EINVAL;
+
 	sba = talloc_zero(tall_pcu_ctx, struct gprs_rlcmac_sba);
 	if (!sba)
 		return -ENOMEM;
 
-	if (!gsm48_ta_is_valid(ta))
-		return -EINVAL;
-
 	for (trx = 0; trx < 8; trx++) {
 		for (ts = 7; ts >= 0; ts--) {
 			pdch = &m_bts.bts_data()->trx[trx].pdch[ts];