valid: Don't add ids when validating entity content

The id table shouldn't reference ids in entities. The id will be created
when expanding the entity.

Probably regressed with d025cfbb.

Note that we still register ids in entities if entities are not
replaced. This is required to make IDREF checks work.

Fixes #974.

include/private/parser.h

@@ -22,6 +22,10 @@
  * Set if the validation is enabled.
  */
 #define XML_VCTXT_VALIDATE (1u << 2)
+/**
+ * Set when parsing entities.
+ */
+#define XML_VCTXT_IN_ENTITY (1u << 3)
 
 /*
  * TODO: Rename to avoid confusion with xmlParserInputFlags