rb_enc_interned_str: handle autoloaded encodings

	If called with an autoloaded encoding that was not yet
	initialized, `rb_enc_interned_str` would crash with
	a NULL pointer exception.

	See: https://github.com/ruby/ruby/pull/4119#issuecomment-800189841
	---
	 encoding.c                        | 28 ++++++++++++----------------
	 ext/-test-/string/depend          |  3 +++
	 ext/-test-/string/fstring.c       | 15 +++++++++++++++
	 internal/encoding.h               |  3 +++
	 string.c                          |  4 ++++
	 test/-ext-/string/test_fstring.rb | 16 ++++++++++++++++
	 6 files changed, 53 insertions(+), 16 deletions(-)