typefindfunctions: Fix WebVTT format detection
If WebVTT file consists of "WebVTT" header without body, the file size can be smaller than 10 bytes.
Moved from gst-plugins-base!1264 (closed)
If WebVTT file consists of "WebVTT" header without body, the file size can be smaller than 10 bytes.
Moved from gst-plugins-base!1264 (closed)