fix SPIFFS_readdir setting errno to SPIFFS_VIS_END

When iteration finishes, spiffs_obj_lu_find_entry_visitor returns
SPIFFS_VIS_END. Previously, SPIFFS_readdir function used to set
errno to this internal error value when readdir reached the end.
Handle this case and add a check into the regression test.

src/spiffs_hydrogen.c

@@ -1121,6 +1121,8 @@ struct spiffs_dirent *SPIFFS_readdir(spiffs_DIR *d, struct spiffs_dirent *e) {
     d->entry = entry + 1;
     e->obj_id &= ~SPIFFS_OBJ_ID_IX_FLAG;
     ret = e;
+  } else if (res == SPIFFS_VIS_END) {
+    // end of iteration
   } else {
     d->fs->err_code = res;
   }

src/test/test_hydrogen.c

@@ -439,6 +439,7 @@ TEST(list_dir)
 #endif
     }
   }
+  TEST_CHECK_EQ(SPIFFS_errno(FS), SPIFFS_OK);
   SPIFFS_closedir(&d);
 
   TEST_CHECK(found == file_cnt);