[Bug #20322] Fix rb_enc_interned_str_cstr null encoding

	The documentation for `rb_enc_interned_str_cstr` notes that `enc` can be
	a null pointer, but this currently causes a segmentation fault when
	trying to autoload the encoding. This commit fixes the issue by checking
	for NULL before calling `rb_enc_autoload`.

	[Bug #19902] Update the coderange regarding the changed region

	---
	 ext/-test-/string/set_len.c       | 10 ++++++++++
	 string.c                          | 27 +++++++++++++++++++++++++++
	 test/-ext-/string/test_set_len.rb | 29 +++++++++++++++++++++++++++++
	 3 files changed, 66 insertions(+)

[Misc #18891]

This commit adds support for embedded strings with variable capacity and
uses Variable Width Allocation to allocate strings.

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