search_nonascii(): Replace UB pointer cast with memcpy

	Casting a pointer to create an unaligned one is undefined behavior in C
	standards. Use memcpy to express the unaligned load instead to play by
	the rules.

	Practically, this yields the same binary output in many situations
	while fixing the crash in [Bug #21715].

	Check negative integer underflow

	Many of Oniguruma functions need valid encoding strings

	[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`.

This reverts commit 6b73406833dd22e489114fa77c1c80c4b7af2ed0.

This reverts commit a54c717c7a74b91a3cdf20742c355e3ea42052d1.

This reverts commit 715633ba6e982dc5404abeafc5246c31af92ac10.

	[Bug #20292] Truncate embedded string to new capacity

	Fix -Wsign-compare on String#initialize
	MIME-Version: 1.0
	Content-Type: text/plain; charset=UTF-8
	Content-Transfer-Encoding: 8bit

	../string.c:1886:57: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘long int’ [-Wsign-compare]
	 1886 |                 if (STR_EMBED_P(str)) RUBY_ASSERT(osize <= str_embed_capa(str));
	      |                                                         ^~