lavu: make sure av_pix_fmt_desc_next returns a valid pix fmt.

This is required because there are some "holes" in the list for
compatibility with the fork.

The commit also removes the now unecessary check from cmdutils.

Found-by: wm4

cmdutils.c

@@ -1195,8 +1195,6 @@ int show_pix_fmts(void *optctx, const char *opt, const char *arg)
 
     while ((pix_desc = av_pix_fmt_desc_next(pix_desc))) {
         enum AVPixelFormat pix_fmt = av_pix_fmt_desc_get_id(pix_desc);
-        if(!pix_desc->name)
-            continue;
         printf("%c%c%c%c%c %-16s       %d            %2d\n",
                sws_isSupportedInput (pix_fmt)      ? 'I' : '.',
                sws_isSupportedOutput(pix_fmt)      ? 'O' : '.',

libavutil/pixdesc.c

@@ -1757,8 +1757,11 @@ const AVPixFmtDescriptor *av_pix_fmt_desc_next(const AVPixFmtDescriptor *prev)
 {
     if (!prev)
         return &av_pix_fmt_descriptors[0];
-    if (prev - av_pix_fmt_descriptors < FF_ARRAY_ELEMS(av_pix_fmt_descriptors) - 1)
-        return prev + 1;
+    while (prev - av_pix_fmt_descriptors < FF_ARRAY_ELEMS(av_pix_fmt_descriptors) - 1) {
+        prev++;
+        if (prev->name)
+            return prev;
+    }
     return NULL;
 }