fftools/ffmpeg_filter: Avoid DynBuf-API for writing strings

It is not really natural, it requires internal allocations
of its own and its error handling is horrible (i.e. the implicit
(re)allocations here are unchecked).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

fftools/ffmpeg_filter.c

@@ -201,17 +201,15 @@ static char *describe_filter_link(FilterGraph *fg, AVFilterInOut *inout, int in)
     AVFilterContext *ctx = inout->filter_ctx;
     AVFilterPad *pads = in ? ctx->input_pads  : ctx->output_pads;
     int       nb_pads = in ? ctx->nb_inputs   : ctx->nb_outputs;
-    AVIOContext *pb;
+    char *res;
-    uint8_t *res = NULL;
 
-    if (avio_open_dyn_buf(&pb) < 0)
-        exit_program(1);
-
-    avio_printf(pb, "%s", ctx->filter->name);
     if (nb_pads > 1)
-        avio_printf(pb, ":%s", avfilter_pad_get_name(pads, inout->pad_idx));
+        res = av_strdup(ctx->filter->name);
-    avio_w8(pb, 0);
+    else
-    avio_close_dyn_buf(pb, &res);
+        res = av_asprintf("%s:%s", ctx->filter->name,
+                          avfilter_pad_get_name(pads, inout->pad_idx));
+    if (!res)
+        exit_program(1);
     return res;
 }