eolian: always validate entire staging area

This is necessary even when parsing a single file because there
may be parsed results directly in the staging area introduced by
doc references, those wouldn't get correctly validated and would
be left in an inconsistent and unusable state.

src/lib/eolian/eolian_database.c

@@ -1032,7 +1032,7 @@ eolian_state_file_parse(Eolian_State *state, const char *filepath)
    if (!_parse_deferred(ret))
      return NULL;
    _merge_units(ret);
-   if (!database_validate(ret))
+   if (!database_validate(&state->staging.unit))
      return NULL;
    _merge_staging(state);
    return ret;