Pekka Paalanen authored Dec 02, 2019 and Jonas Ådahl committed Jan 16, 2020

To mirror what happens in meta_onscreen_native_swap_buffers_with_damage(), warn
here too if next_fb is not NULL. This makes it clear to the reader of what the
expectations are inside this function.

Ensuring next_fb is NULL as the first thing in the function will make all error
paths equal: no longer some failures reset next_fb while others don't. Removing
such special cases should reduce surprises.

!810