uninorm/composition.c:75:22: runtime error

[Simon Josefsson]

Hi.  I encountered the following runtime error when running libidn2
self-tests (log in [1]) with ASAN/UBSAN checks [2] and recent gnulib.

uninorm/composition.c:75:22: runtime error: implicit conversion from type 
'unsigned int' of value 218 (32-bit, unsigned) to type 'char' changed the value 
to -38 (8-bit, signed)

The system has libunistring 0.9.10 installed, which could affect this,
since the way libidn2 uses gnulib's libunistring modules may be
incorrect -- in particular, it forgot to link with gnulib's libunistring
replacement modules when the system has libunistring -- however this
problem started to appear when I attempted [3] to fix that issue.  That
suggests the unistring code always would trigger this problem, but it
was hidden by the libidn2 build problem that made it use the system
libunistring instead.

The uninorm/composition.c code looks like this:

ucs4_t
uc_composition (ucs4_t uc1, ucs4_t uc2)
{
...
          char codes[6];
...
          codes[2] = uc1 & 0xff;

Thoughts?  The code has looked the same for a long time.

/Simon

[1] https://gitlab.com/libidn/libidn2/-/jobs/2163463557
[2] https://gitlab.com/libidn/libidn2/-/blob/master/.gitlab-ci.yml#L386
[3] 
https://gitlab.com/libidn/libidn2/-/commit/797e2ac6badcd80bac0b74191309f20356e48a50

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