vaapiencode: don't crash on NULL encoder on _finish().

vaapiencode: don't crash on NULL encoder on _finish().

Don't try to destroy an encoder, in GstVideoEncoder::finish() handler,
if it was not created in the first place. Return "not-negotiated" error
since this means we did not even reach GstVideoEncoder::set_format(),
where the encoder could have been created.

This fixes a crash when the vaapiencode_* plug-in elements get deallocated
and that we failed to negotiate either pad.

https://bugzilla.gnome.org/show_bug.cgi?id=719704