Assertion when printing trees

[jktjkt]

It's never OK to assert. Claude hit this while using it as a library, here's a small shell reproducer.

$ yanglint -f tree libyang/modules/ietf-yang-library-augmentedby\@2025-05-28.yang libyang/modules/ietf-yang-library-semver\@2025-09-29.yang 
module: ietf-yang-library-augmentedby

  augment /yanglib:yang-library/yanglib:module-set/yanglib:module:
    +---u augmented-by
  augment /yanglib:modules-state/yanglib:module:
    +---u augmented-by

module: ietf-yang-library-semver

  augment /yanglib:yang-library/yanglib:module-set/yanglib:module:
    +---u module-version
  augment /yanglib:yang-library/yanglib:module-set/yanglib:module/yanglib:submodule:
    +---u module-version
  augment /yanglib:yang-library/yanglib:module-set/yanglib:import-only-module:
    +---u module-version
  augment /yanglib:yang-library/yanglib:module-set/yanglib:import-only-module/yanglib:submodule:
yanglint: libyang/src/plugins_exts.c:585: LY_ERR lyplg_ext_parsed_get_storage(const struct lysc_ext_instance *, int, uint32_t, const void **): Assertion `extp' failed.