avutil/hdr_dynamic_metadata: fix from_t35 NULL return and version doc

av_dynamic_hdr_plus_from_t35() returned AVERROR(ENOMEM) when passed a
NULL AVDynamicHDRPlus. A NULL output struct is invalid API usage by the
caller, not an allocation failure, and nothing is allocated on that
path. av_dynamic_hdr_smpte2094_app5_from_t35() already returns
AVERROR(EINVAL) for the same condition; make the two parsers agree.

The documentation for AVDynamicHDRPlus.application_version said the
value shall be set to 0, which contradicts both sides of the
implementation: av_dynamic_hdr_plus_to_t35() ignores the field and
always writes 1, the value CTA-861-H specifies for the HDR10+ payload,
and av_dynamic_hdr_plus_from_t35() sets the field from the parsed
payload rather than validating it against 0. No FFmpeg code assigns
this field, so document the contract the implementation provides.

Signed-off-by: marcos ashton <marcosashiglesias@gmail.com>

libavutil/hdr_dynamic_metadata.c

@@ -64,7 +64,7 @@ int av_dynamic_hdr_plus_from_t35(AVDynamicHDRPlus *s, const uint8_t *data,
     int ret;
 
     if (!s)
-        return AVERROR(ENOMEM);
+        return AVERROR(EINVAL);
 
     if (size > AV_HDR_PLUS_MAX_PAYLOAD_SIZE)
         return AVERROR(EINVAL);

libavutil/hdr_dynamic_metadata.h

@@ -248,7 +248,11 @@ typedef struct AVDynamicHDRPlus {
 
     /**
      * Application version in the application defining document in ST-2094
-     * suite. The value shall be set to 0.
+     * suite. CTA-861-H sets this to 1 for the HDR10+ payload carried in a
+     * Rec. ITU-T T.35 message.
+     *
+     * av_dynamic_hdr_plus_from_t35() sets this field from the parsed payload.
+     * av_dynamic_hdr_plus_to_t35() ignores it and always writes 1.
      */
     uint8_t application_version;