fftools/ffmpeg_filter: only use fallback parameters when necessary

With complex filtergraphs it can happen that the filtergraph is
unconfigured because some other filter than the one we just got EOF on
is missing parameters.

Make sure that the fallback parametes for a given input are only used
when that input is unconfigured.

fftools/ffmpeg_filter.c

@@ -1521,6 +1521,7 @@ int ifilter_send_eof(InputFilter *ifilter, int64_t pts, AVRational tb)
         if (ret < 0)
             return ret;
     } else {
+        if (ifilter->format < 0) {
             // the filtergraph was never configured, use the fallback parameters
             ifilter->format                 = ifp->fallback.format;
             ifilter->sample_rate            = ifp->fallback.sample_rate;
@@ -1532,6 +1533,7 @@ int ifilter_send_eof(InputFilter *ifilter, int64_t pts, AVRational tb)
                                          &ifp->fallback.ch_layout);
             if (ret < 0)
                 return ret;
+        }
 
         if (ifilter->format < 0 && (ifilter->type == AVMEDIA_TYPE_AUDIO || ifilter->type == AVMEDIA_TYPE_VIDEO)) {
             av_log(NULL, AV_LOG_ERROR, "Cannot determine format of input stream %d:%d after EOF\n", ifilter->ist->file_index, ifilter->ist->st->index);