flvmux: Avoid crash when best pad gets flushed
flvmux: Avoid crash when best pad gets flushed

The 'best' pad might receive a flush event between us picking it and us
popping the buffer. In this case, the buffer will be missing.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/711>

GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good