avcodec/hnm4video: allocate frame only when theres a coded frame

Fixes memleak
Fixes: asan_heap-oob_e76bdf_2224_MOTHOO.HNM

This patch also removes the setting of palette_has_changed,
which was set on a frame that was never returned

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
