vf-metadata: fix handling of NULL metadata

lavfi would segfault due to a NULL dereference if it was asked for its
metadata and none had been allocated (oops). This happens for libav
which has no concept of filter metadata.

player/command.c

@@ -950,19 +950,22 @@ static int mp_property_vf_metadata(m_option_t *prop, int action, void *arg,
         char *rem;
         m_property_split_path(ka->key, &key, &rem);
         struct mp_tags vf_metadata;
-        if (vf_control_by_label(vf, VFCTRL_GET_METADATA, &vf_metadata, key)
-                == CONTROL_UNKNOWN)
-            return M_PROPERTY_UNKNOWN;
-
-        if (strlen(rem)) {
+        switch (vf_control_by_label(vf, VFCTRL_GET_METADATA, &vf_metadata, key)) {
+        case CONTROL_NA:
+          return M_PROPERTY_UNAVAILABLE;
+        case CONTROL_UNKNOWN:
+          return M_PROPERTY_UNKNOWN;
+        case CONTROL_OK:
+          if (strlen(rem)) {
             struct m_property_action_arg next_ka = *ka;
             next_ka.key = rem;
             return tag_property(prop, M_PROPERTY_KEY_ACTION, &next_ka,
                                 &vf_metadata);
-        } else {
+          } else {
             return tag_property(prop, ka->action, ka->arg, &vf_metadata);
-        }
-        return M_PROPERTY_OK;
+          }
+	  return M_PROPERTY_OK;
+	}
     }
     }
     return M_PROPERTY_NOT_IMPLEMENTED;

video/filter/vf_lavfi.c

@@ -323,10 +323,13 @@ static int control(vf_instance_t *vf, int request, void *data)
     case VFCTRL_SEEK_RESET:
         reset(vf);
         return CONTROL_OK;
-    case VFCTRL_GET_METADATA:{
+    case VFCTRL_GET_METADATA:
+      if (vf->priv && vf->priv->metadata) {
         *(struct mp_tags*) data = *vf->priv->metadata;
         return CONTROL_OK;
-    }
+      } else {
+	return CONTROL_NA;
+      }
     }
     return CONTROL_UNKNOWN;
 }