fftools/ffmpeg_opt: Fix leak on error

Fixes Coverity issue #1539097.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

fftools/ffmpeg_opt.c

@@ -497,7 +497,7 @@ static int opt_map_channel(void *optctx, const char *opt, const char *arg)
 
     ret = GROW_ARRAY(o->audio_channel_maps, o->nb_audio_channel_maps);
     if (ret < 0)
-        return ret;
+        goto end;
 
     m = &o->audio_channel_maps[o->nb_audio_channel_maps - 1];
 
@@ -559,11 +559,13 @@ static int opt_map_channel(void *optctx, const char *opt, const char *arg)
         }
 
     }
+    ret = 0;
+end:
     av_free(mapchan);
-    return 0;
+    return ret;
 fail:
-    av_free(mapchan);
-    return AVERROR(EINVAL);
+    ret = AVERROR(EINVAL);
+    goto end;
 }
 #endif