We were calling gst_v4l2_is_buffer_valid() before and inside
gst_v4l2_buffer_pool_qbuf() as we needed to access the group. The second
check failed since the writability of the buffer get inherited from the
GstMemory, which lead to pipeline failure. As we cannot avoid the extra
ref, it would be racy otherwise, just pass the group to _dbuf() so it
does not have to call gst_v4l2_is_buffer_valid() again.
https://bugzilla.gnome.org/show_bug.cgi?id=796692
38b68cbbd7