dict.c: Free non-strduped av_dict_set arguments on error.

Unfortunately this was not explicitly documented and thus
might be risky.
But all uses I could find in FFmpeg and one in VLC had a memleak
in these cases, and I could not find any that relied on the previous
behaviour.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
doc/filters: fix Shwo/Show typo
avutil/pixelutils: faster pixelutils_sad_16x16

501 to 439 decicycles.

See 45c7f3997ea11c3d1007b2126b1c0049a8c27105.