fftools/ffmpeg_opt: fix mismatching negative maps

Fixes:  -f lavfi -i testsrc2=size=128x128:rate=1:d=1   -filter_complex '[0:v]scale=64:64[vout]'   -map '[vout]'   -map -0:v   -f null -
Previously  -0:v matched [vout] apparently

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

fftools/ffmpeg_opt.c

@@ -645,6 +645,7 @@ static int opt_map(void *optctx, const char *opt, const char *arg)
             for (i = 0; i < o->nb_stream_maps; i++) {
                 m = &o->stream_maps[i];
                 if (file_idx == m->file_index &&
+                    !m->linklabel &&
                     m->stream_index >= 0 &&
                     m->stream_index < input_files[m->file_index]->nb_streams &&
                     stream_specifier_match(&ss,