can-host-socketcan: Fix crash when 'if' option is not set

Fix the following crash:

  $ qemu-system-x86_64 -object can-host-socketcan,id=obj0
  Segmentation fault (core dumped)

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Acked-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Message-Id: <20201008202713.1416823-3-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

net/can/can_socketcan.c

@@ -194,6 +194,11 @@ static void can_host_socketcan_connect(CanHostState *ch, Error **errp)
     struct sockaddr_can addr;
     struct ifreq ifr;
 
+    if (!c->ifname) {
+        error_setg(errp, "'if' property not set");
+        return;
+    }
+
     /* open socket */
     s = qemu_socket(PF_CAN, SOCK_RAW, CAN_RAW);
     if (s < 0) {