MATCH_PER_STREAM_OPT iterates over all options of a given
OptionDef and tests whether they apply to the current stream;
if so, they are set to ost->apad, otherwise, the code errors
out. If no error happens, ost->apad is av_strdup'ed in order
to take ownership of this pointer.

But this means that setting it originally was premature,
as it leads to double-frees when an error happens lateron.
This can simply be reproduced with
ffmpeg -filter_complex anullsrc  -apad bar -apad:n baz -f null -
This is a regression since 83ace80b.

Fix this by using a temporary variable instead of directly
setting ost->apad. Also only strdup the string if it actually
is != NULL.
Reviewed-by: Marth64 <marth64@proxyid.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>