fftools/ffmpeg_mux_init: Fix double-free on error

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 83ace80bfd.

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>

fftools/ffmpeg_mux_init.c

@@ -832,6 +832,7 @@ static int new_stream_audio(Muxer *mux, const OptionsContext *o,
         int channels = 0;
         char *layout = NULL;
         char *sample_fmt = NULL;
+        const char *apad = NULL;
 
         MATCH_PER_STREAM_OPT(audio_channels, i, channels, oc, st);
         if (channels) {
@@ -854,8 +855,12 @@ static int new_stream_audio(Muxer *mux, const OptionsContext *o,
 
         MATCH_PER_STREAM_OPT(audio_sample_rate, i, audio_enc->sample_rate, oc, st);
 
-        MATCH_PER_STREAM_OPT(apad, str, ost->apad, oc, st);
-        ost->apad = av_strdup(ost->apad);
+        MATCH_PER_STREAM_OPT(apad, str, apad, oc, st);
+        if (apad) {
+            ost->apad = av_strdup(apad);
+            if (!ost->apad)
+                return AVERROR(ENOMEM);
+        }
     }
 
     return 0;