up_session: Silence coverity warning

Coverity complains about a supposedly missing NULL check for the pointer 'pdr', that check
however happens before the function in question is called.

It makes sense to be consistent inside the function, so remove the NULL check before calling
'pdr_del', because we don't NULL check anywhere else in the function either.

Fixes: Coverity scan CID#307494
Change-Id: Ia33e4211b4a24abc87c3c2ceffe807ca3322f29d

src/osmo-upf/up_session.c

@@ -556,8 +556,7 @@ static struct pdr *pdr_upd(struct pdr *pdr,
 	return pdr;
 
 nack_resp:
-	if (pdr)
-		pdr_del(pdr);
+	pdr_del(pdr);
 	if (!*offending_ie_present) {
 		*offending_ie = OSMO_PFCP_IEI_UPD_PDR;
 		*offending_ie_present = true;