Fix utf16_string_value_ptr() for TruffleRuby

[eregon]

• TruffleRuby uses TruffleString which requires all UTF-16 strings to
  always have an even bytesize.
• The code here ensures the UTF-16 string always ends up with 2 \x00
  bytes, so append them explicitly to have an even bytesize instead of
  relying on RSTRING_PTR() adding the second \x00 byte.
• Fixes Very recent UTF-16 issue? Not.sure. truffleruby/truffleruby#2704

This fixes the CI on TruffleRuby.

[eregon]

From truffleruby/truffleruby#2704 (comment)
I wonder why sqlite3_open16 is used, instead of just always using sqlite3_open_v2 and transcoding the filename from UTF-16 to UTF-8 (which should works fine as long as the string is not broken).
It feels like we could remove the usage of sqlite3_open16 fairly easily and a bunch of code with it.

Maybe it was added in 1.8 days where there was no good encoding/transcoding support in Ruby?

[eregon]

Ah, the side effect of sqlite3_open16 is to set the database encoding to UTF-16 instead of UTF-8 (https://www.sqlite.org/c3ref/open.html).
And I'm not sure there is a way to set "the default encoding for databases" after sqlite3_open_v2.
So this PR makes most sense and we can't remove sqlite3_open16 too easily if users want a UTF-16 "default encoding for databases".

[eregon]

FWIW CRuby master seems to have TERM_LEN logic to ensure there are rb_enc_mbminlen() \x00 bytes at the end, but that's currently not implemented on TruffleRuby, and I'm not sure since which version of CRuby that's done.

[eregon]

but that's currently not implemented on TruffleRuby

That seems easy to fix, so I have an incoming fix for that in TruffleRuby.
So if we're happy to rely on Ruby having enough \x00 bytes, then we can replace utf16_string_value_ptr() with RSTRING_PTR().
But CRuby 1.9.2 does not seem to have the TERM_LEN logic, e.g. https://github.com/ruby/ruby/blob/ruby_1_9_2/string.c#L410
TERM_LEN was added in CRuby 2.1: ruby/ruby@b271a8c

[flavorjones]

@eregon Can you please add a test for this behavior?

[flavorjones]

Ah, I see, it fails when truffleruby uses packaged libraries, but not when it uses system libraries.

I see the commment at truffleruby/truffleruby#2704 (comment) explaining the source of the problem.

No worries, I'll work through it.

[flavorjones]

Thank you!