rcv_resource_request(): Validate expected message before MS lookup & allocation

There's no sense if doing the lookup and allocation if the message is
not expected, it will be unrefed (freed) afterwards anyway.
Moreover, this way we avoid doing stuff for the WIP code paths which act
on different request ID than TLLI.

Change-Id: I4be8858230a2eebdb33260093d082a005cb9fcd4
diff --git a/src/pdch.cpp b/src/pdch.cpp
index 369406f..defb6b2 100644
--- a/src/pdch.cpp
+++ b/src/pdch.cpp
@@ -644,10 +644,16 @@
 	struct gprs_rlcmac_sba *sba;
 	int rc;
 	struct gprs_rlcmac_bts *bts = trx->bts;
+	struct pdch_ulc_node *item;
+
+	if (!(item = pdch_ulc_get_node(ulc, fn))) {
+		LOGPDCH(this, DRLCMAC, LOGL_NOTICE, "FN=%u PKT RESOURCE REQ: "
+			"UL block not reserved\n", fn);
+		return;
+	}
 
 	if (request->ID.UnionType) {
 		struct gprs_rlcmac_ul_tbf *ul_tbf = NULL;
-		struct pdch_ulc_node *item;
 		uint32_t tlli = request->ID.u.TLLI;
 
 		GprsMs *ms = bts_ms_by_tlli(bts, tlli, GSM_RESERVED_TMSI);
@@ -659,12 +665,6 @@
 		/* Keep the ms, even if it gets idle temporarily */
 		ms_ref(ms);
 
-		if (!(item = pdch_ulc_get_node(ulc, fn))) {
-			LOGPDCH(this, DRLCMAC, LOGL_NOTICE, "FN=%u PKT RESOURCE REQ: "
-				"UL block not reserved\n", fn);
-			goto return_unref;
-		}
-
 		switch (item->type) {
 		case PDCH_ULC_NODE_TBF_USF:
 			/* Is it actually valid for an MS to send a PKT Res Req during USF? */