BUG: seeking to last frame caused EoF in pyav

[FirefoxMetzger]

I noticed a bug in pyav that would cause a EoF exception when you try to read only the last frame of the image, e.g. something like

import imageio.v3 as iio

props = iio.improps("imageio:cockatoo.mp4", plugin="pyav")
frame = iio.imread("imageio:cockatoo.mp4", index=props.shape[0]-1, plugin="pyav")

This was caused be me getting the decoder after seeking via container.decode instead of reusing the decoder I used for seeking. I thought the object was shared, but apparently it is not ... this PR fixes that.