fftools/cmdutils: Use avfilter_pad_count() for AVFilter's number of pads

Besides being nicer code this also has the advantage of not making
assumptions about the internal implementation: While it is documented
that the AVFilter.inputs and AVFilter.outputs arrays are terminated
by a zeroed sentinel, one is not allowed to infer that one can just
check avfilter_pad_get_name(padarray, i) to see whether one has reached
the sentinel:
It could be that the pointer to the string is contained
in a different structure than AVFilterPad that needs to be accessed
first: return pad->struct->string.
It could be that for small strings an internal buffer in
AVFilterPad is used (to avoid a relocation) whereas for longer strings
an external string is used; this is useful to avoid relocations:
return pad->string_ptr ? pad->string_ptr : pad->interal_string
Or it could be that the name has a default value:
return pad->name ? pad->name : "default"
(This actually made sense for us because the name of most of our
AVFilterPads is just "default"; doing so would save lots of relocations.)

The only thing one is allowed to infer from the existence of the
sentinel is that one is allowed to use avfilter_pad_count() to get
the number of pads. Therefore it is used.

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

fftools/cmdutils.c

@@ -1707,12 +1707,14 @@ int show_filters(void *optctx, const char *opt, const char *arg)
     while ((filter = av_filter_iterate(&opaque))) {
         descr_cur = descr;
         for (i = 0; i < 2; i++) {
+            unsigned nb_pads;
             if (i) {
                 *(descr_cur++) = '-';
                 *(descr_cur++) = '>';
             }
             pad = i ? filter->outputs : filter->inputs;
-            for (j = 0; pad && avfilter_pad_get_name(pad, j); j++) {
+            nb_pads = avfilter_pad_count(pad);
+            for (j = 0; j < nb_pads; j++) {
                 if (descr_cur >= descr + sizeof(descr) - 4)
                     break;
                 *(descr_cur++) = get_media_type_char(avfilter_pad_get_type(pad, j));