lavfi/show_palette: fix memory leak

Recent commits 6aaac24d72a7da631173209841a3944fcb4a3309 and
3835554bf8ed78539a3492c239f979c0ab03a15f made progress towards cleaning
up usage of the formats API, and in particular fixed possible NULL pointer
dereferences.

This commit addresses the issue of possible resource leaks when some intermediate
call fails. Unfortunately, even leaving aside this subtle intermediate
failure aspect, commit 8087632027d755cd32ccc9e91ea025e276197055 was only
partially successful in addressing memleaks. Hopefully, this commit
fixes the issue completely.

Tested with valgrind --leak-check=full --show-leak-kinds=all, and manual simulation
of malloc/realloc failures.

Fixes: CID 1270818.

Reviewed-by: Clément Bœsch <u@pkh.me>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
