util: netdev: Don't crash in virNetDevSetIPAddress if @peer is NULL

VIR_SOCKET_ADDR_VALID dereferences the pointer, thus if we pass NULL
into virNetDevSetIPAddress it crashes. Regression introduced by
b3d06987.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1325120

src/util/virnetdev.c

@@ -1129,7 +1129,7 @@ int virNetDevSetIPAddress(const char *ifname,
     unsigned int recvbuflen;
 
     /* The caller needs to provide a correct address */
-    if (VIR_SOCKET_ADDR_FAMILY(addr) == AF_INET && !VIR_SOCKET_ADDR_VALID(peer)) {
+    if (VIR_SOCKET_ADDR_FAMILY(addr) == AF_INET && peer && !VIR_SOCKET_ADDR_VALID(peer)) {
         /* compute a broadcast address if this is IPv4 */
         if (VIR_ALLOC(broadcast) < 0)
             return -1;
@@ -1445,7 +1445,7 @@ int virNetDevSetIPAddress(const char *ifname,
     if (!(addrstr = virSocketAddrFormat(addr)))
         goto cleanup;
 
-    if (VIR_SOCKET_ADDR_VALID(peer) && !(peerstr = virSocketAddrFormat(peer)))
+    if (peer && VIR_SOCKET_ADDR_VALID(peer) && !(peerstr = virSocketAddrFormat(peer)))
         goto cleanup;
 
     /* format up a broadcast address if this is IPv4 */