[mod_webdav] fix theoretical NULL dereference

fix theoretical NULL dereference identified by Coverity Scan

possible for PROPFIND with specific atypical choices in lighttpd.conf:
- possible for getcontenttype if no content type matches resource
  and no default type configured in lighttpd.conf
- possible for getetag if etag disabled in lighttpd.conf

src/mod_webdav.c

@@ -3172,7 +3172,18 @@ webdav_propfind_resource_props (const webdav_propfind_bufs * const restrict pb)
                 continue;
 
             /*(error obtaining prop if reached)*/
-            webdav_xml_prop(pb->b_404, prop, NULL, 0);
+            if (prop->name)
+                webdav_xml_prop(pb->b_404, prop, NULL, 0);
+            else {
+                const struct live_prop_list *list = live_properties;
+                while (0 != list->len && (uint32_t)list->pnum != prop->namelen)
+                    ++list;
+                if (0 != list->len) { /*(list->pnum == prop->namelen)*/
+                    webdav_property_name lprop =
+                      { prop->ns, list->prop, prop->nslen, list->len };
+                    webdav_xml_prop(pb->b_404, &lprop, NULL, 0);
+                }
+            }
         }
     }