filter-mirror: segfault when specifying non existent device

When using filter-mirror like the example below where the interface
'ndev0' does not exist on the host, QEMU crashes into segmentation
fault.

 $ qemu-system-x86_64 -S -machine pc -netdev user,id=ndev0 -object filter-mirror,id=test-object,netdev=ndev0

This happens because the function filter_mirror_setup() does not check
if the device actually exists and still keep on processing calling
qemu_chr_find(). This patch fixes this issue.

Signed-off-by: Eduardo Otubo <otubo@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

net/filter-mirror.c

@@ -213,6 +213,12 @@ static void filter_mirror_setup(NetFilterState *nf, Error **errp)
     MirrorState *s = FILTER_MIRROR(nf);
     Chardev *chr;
 
+    if (s->outdev == NULL) {
+        error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND, "filter-mirror parameter"\
+                  " 'outdev' cannot be empty");
+        return;
+    }
+
     chr = qemu_chr_find(s->outdev);
     if (chr == NULL) {
         error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND,