videoio: fix isOpened() method for legacy APIs, fix "index" API list

modules/videoio/src/cap.cpp

@@ -169,7 +169,7 @@ bool VideoCapture::isOpened() const
 {
     if (!icap.empty())
         return icap->isOpened();
-    return !icap.empty();  // legacy interface doesn't support closed files
+    return !cap.empty();  // legacy interface doesn't support closed files
 }
 
 void VideoCapture::release()

modules/videoio/src/videoio_registry.cpp

@@ -288,7 +288,7 @@ namespace videoio_registry {
 
 std::vector<VideoBackendInfo> getAvailableBackends_CaptureByIndex()
 {
-    const std::vector<VideoBackendInfo> result = VideoBackendRegistry::getInstance().getAvailableBackends_CaptureByFilename();
+    const std::vector<VideoBackendInfo> result = VideoBackendRegistry::getInstance().getAvailableBackends_CaptureByIndex();
     return result;
 }
 std::vector<VideoBackendInfo> getAvailableBackends_CaptureByFilename()