tag: added tag_is_defined()

tag_is_defined() checks whether there is any information in the tag
object.

src/tag.h

@@ -109,6 +109,15 @@ tag_is_empty(const struct tag *tag)
 	return tag->numOfItems == 0;
 }
 
+/**
+ * Returns true if the tag contains any information.
+ */
+static inline bool
+tag_is_defined(const struct tag *tag)
+{
+	return !tag_is_empty(tag) || tag->time >= 0;
+}
+
 /**
  * Returns the first value of the specified tag type, or NULL if none
  * is present in this tag object.