flvdemux: Avoid integer overflow on invalid CTS
flvdemux: Avoid integer overflow on invalid CTS

If the CTS is negative an would lead to a negtive PTS, clip
the CTS so the PTS will be 0.

https://bugzilla.gnome.org/show_bug.cgi?id=787795

StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good