Add missing NULL check to opus_tags_parse().

According to the API, you can pass in a NULL OpusTags object to
 simply check if the comment packet is valid, without storing the
 parsed results.
However, the additions to store binary metadata in commit
 0221ca95 did not check for this.

Fixes Coverity CID 149873.

src/info.c

@@ -205,10 +205,12 @@ static int opus_tags_parse_impl(OpusTags *_tags,
   }
   if(len>0&&(_data[0]&1)){
     if(len>(opus_uint32)INT_MAX)return OP_EFAULT;
-    _tags->user_comments[ncomments]=(char *)_ogg_malloc(len);
-    if(OP_UNLIKELY(_tags->user_comments[ncomments]==NULL))return OP_EFAULT;
-    memcpy(_tags->user_comments[ncomments],_data,len);
-    _tags->comment_lengths[ncomments]=(int)len;
+    if(_tags!=NULL){
+      _tags->user_comments[ncomments]=(char *)_ogg_malloc(len);
+      if(OP_UNLIKELY(_tags->user_comments[ncomments]==NULL))return OP_EFAULT;
+      memcpy(_tags->user_comments[ncomments],_data,len);
+      _tags->comment_lengths[ncomments]=(int)len;
+    }
   }
   return 0;
 }