[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`.

Already these had been announced to be removed in 3.2.

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

This removes the related tests, and puts the related specs behind
version guards.  This affects all code in lib, including some
libraries that may want to support older versions of Ruby.

"in void context" by assigning the result to a dummy variable.

The buffer deduplication codepath in rb_fstring can be used to free the buffer
of shared string roots, which leads to use-after-free.

Introudce a new flag to tag strings that at one point have been a shared root.
Check for it in rb_fstring to avoid freeing buffers that are shared by
multiple strings. This change is based on nobu's idea in [ruby-core:94838].

The included test case test for the sequence of calls to internal functions
that lead to this bug. See attached ticket for Ruby level repros.

[Bug #16151]

[Bug #14194]

* test/-ext-/string/test_fstring.rb (test_singleton_class): fix
  expected exception class.  [ruby-dev:49867] [Bug #12923]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56754 b2dd03c8-39d4-4d8f-98ff-823fe69b080e