fftools: do not access out of bounds filtergraph

The log message was logged for `filtergraphs[j]` which would cause a
heap buffer overflow in certain circumstances.

Correctly it should be logged for the current filtergraph, so just
use `fg` here.

(cherry picked from commit 5beeb3a1f97d8f6d4076fe83aaf5e2e5871f945e)

fftools/ffmpeg_filter.c

@@ -1408,7 +1408,7 @@ int fg_finalise_bindings(void)
         for (int j = 0; j < fg->nb_outputs; j++) {
             OutputFilter *output = fg->outputs[j];
             if (!output->bound) {
-                av_log(filtergraphs[j], AV_LOG_FATAL,
+                av_log(fg, AV_LOG_FATAL,
                        "Filter %s has an unconnected output\n", output->name);
                 return AVERROR(EINVAL);
             }