Fix POSIX::mblen mbstate_t initialization on threaded perls with glibc

As reported in https://bugs.launchpad.net/bugs/1818953 POSIX::mblen()
is broken on threaded perls with glibc.

  % perl -MPOSIX=mblen -e 'mblen("a", 1)'
  perl: mbrtowc.c:105: __mbrtowc: Assertion `__mbsinit (data.__statep)' failed.
  zsh: abort (core dumped)  perl -MPOSIX=mblen -e 'mblen("a", 1)'

This broke in v5.27.8-134-g6c9ff7e96e which made the function
use mbrlen(3) under the hood on threaded perls.

The problem is initialization of the shift state with

  mbrlen(NULL, 0, &ps));

The glibc documentation for mbrlen(3) at

  https://www.gnu.org/software/libc/manual/html_node/Converting-a-Character.html#Converting-a-Character

does not mention initialization by passing in a null pointer for the
string, only a pointer to a NUL wide character.

   If the next multibyte character corresponds to the NUL wide character,
   the return value is 0. If the next n bytes form a valid multibyte
   character, the number of bytes belonging to this multibyte character
   byte sequence is returned.

Use memset(3) instead for mbstate_t initialization, as suggested in

  https://www.gnu.org/software/libc/manual/html_node/Keeping-the-state.html

with the hope that this is more portable.

While at it, add a few basic test cases. These are in a new file because
they need fresh_perl_is() from test.pl while the existing ones use
Test::More (and conversion of at least posix.t looks way too involved.)

Bug-Ubuntu: https://bugs.launchpad.net/bugs/1818953

Author: ntyni

MANIFEST

@@ -4182,6 +4182,7 @@ ext/POSIX/POSIX.xs		POSIX extension external subroutines
 ext/POSIX/t/export.t		Test @EXPORT and @EXPORT_OK
 ext/POSIX/t/iscrash		See if POSIX isxxx() crashes with threads on Win32
 ext/POSIX/t/math.t		Basic math tests for POSIX
+ext/POSIX/t/mb.t		Multibyte function tests for POSIX
 ext/POSIX/t/posix.t		See if POSIX works
 ext/POSIX/t/sigaction.t		See if POSIX::sigaction works
 ext/POSIX/t/sigset.t		See if POSIX::SigSet works

ext/POSIX/POSIX.xs

@@ -3329,7 +3329,7 @@ mblen(s, n)
 #endif
     CODE:
 #if defined(USE_ITHREADS) && defined(HAS_MBRLEN)
-        PERL_UNUSED_RESULT(mbrlen(NULL, 0, &ps));   /* Initialize state */
+        memset(&ps, 0, sizeof(ps)); /* Initialize state */
         RETVAL = mbrlen(s, n, &ps); /* Prefer reentrant version */
 #else
         RETVAL = mblen(s, n);

ext/POSIX/lib/POSIX.pm

@@ -4,7 +4,7 @@ use warnings;
 
 our ($AUTOLOAD, %SIGRT);
 
-our $VERSION = '1.87';
+our $VERSION = '1.88';
 
 require XSLoader;
 

ext/POSIX/t/mb.t

@@ -0,0 +1,47 @@
+#!./perl
+
+# These tests are in a separate file, because they use fresh_perl_is()
+# from test.pl.
+
+# The mb* functions use the "underlying locale" that is not affected by
+# the Perl one.  So we run the tests in a separate "fresh_perl" process
+# with the correct LC_CTYPE set in the environment.
+
+BEGIN {
+    require Config; import Config;
+    if ($^O ne 'VMS' and $Config{'extensions'} !~ /\bPOSIX\b/) {
+	print "1..0\n";
+	exit 0;
+    }
+    unshift @INC, "../../t";
+    require 'loc_tools.pl';
+    require 'test.pl';
+}
+
+plan tests => 3;
+
+use POSIX qw();
+
+SKIP: {
+    skip("mblen() not present", 3) unless $Config{d_mblen};
+
+    is(&POSIX::mblen("a", &POSIX::MB_CUR_MAX), 1, 'mblen() basically works');
+
+    skip("LC_CTYPE locale support not available", 2)
+      unless locales_enabled('LC_CTYPE');
+
+    my $utf8_locale = find_utf8_ctype_locale();
+    skip("no utf8 locale available", 2) unless $utf8_locale;
+
+    local $ENV{LC_CTYPE} = $utf8_locale;
+    local $ENV{LC_ALL};
+    delete $ENV{LC_ALL};
+
+    fresh_perl_is(
+      'use POSIX; print &POSIX::mblen("\x{c3}\x{28}", &POSIX::MB_CUR_MAX)',
+      -1, {}, 'mblen() recognizes invalid multibyte characters');
+
+    fresh_perl_is(
+     'use POSIX; print &POSIX::mblen("\N{GREEK SMALL LETTER SIGMA}", &POSIX::MB_CUR_MAX)',
+     2, {}, 'mblen() works on UTF-8 characters');
+}