avfilter/tests/filtfmts: Check for existence of formats/channel layouts

Fixes segfaults with filters that either return AVERROR(EAGAIN)
(or another error) or that do not set everything and rely on
filter_query_formats() to set the rest.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/tests/filtfmts.c

@@ -43,7 +43,7 @@ static void print_formats_internal(AVFilterLink **links, const AVFilterPad *pads
 
         if (link->type == AVMEDIA_TYPE_VIDEO) {
             const AVFilterFormats *const fmts = cfg->formats;
-            for (unsigned j = 0; j < fmts->nb_formats; j++) {
+            for (unsigned j = 0; fmts && j < fmts->nb_formats; j++) {
                 printf("%s[%u] %s: fmt:%s\n",
                         inout_string, i, pad_name,
                         av_get_pix_fmt_name(fmts->formats[j]));
@@ -52,12 +52,12 @@ static void print_formats_internal(AVFilterLink **links, const AVFilterPad *pads
             const AVFilterFormats *const fmts = cfg->formats;
             const AVFilterChannelLayouts *const layouts = cfg->channel_layouts;
 
-            for (unsigned j = 0; j < fmts->nb_formats; j++)
+            for (unsigned j = 0; fmts && j < fmts->nb_formats; j++)
                 printf("%s[%u] %s: fmt:%s\n",
                        inout_string, i, pad_name,
                        av_get_sample_fmt_name(fmts->formats[j]));
 
-            for (unsigned j = 0; j < layouts->nb_channel_layouts; j++) {
+            for (unsigned j = 0; layouts && j < layouts->nb_channel_layouts; j++) {
                 char buf[256];
                 av_get_channel_layout_string(buf, sizeof(buf), -1,
                                              layouts->channel_layouts[j]);