downloadbuffer: Check for flush after seek

In gst_download_buffer_wait_for_data(), when a seek is made with perform_seek_to_offset() the qlock is released temporarily. Therefore, the flushing condition can be set during this period and should be checked.

This was not being checked before, causing occasional deadlocks when GST_DOWNLOAD_BUFFER_WAIT_ADD_CHECK() was called.

GST_DOWNLOAD_BUFFER_WAIT_ADD_CHECK() assumes that the caller has already checked that we're not flushing before, since this is done when acquiring the lock; so if we release it temporarily somewhere, we need to check for flush again.

Without that check, the function would keep waiting for the condition variable to be notified before checking for flushing condition again, and that may very well never happen. This was reproduced when during pad deactivation when running WebKit in gdb.