hw/char: replace assert(0) with g_assert_not_reached()

hw/char: replace assert(0) with g_assert_not_reached()

Use of assert(false) can trip spurious control flow warnings from
some versions of GCC (i.e. using -fsanitize=thread with gcc-12):

  error: control reaches end of non-void function [-Werror=return-type]
        default:
            assert(0);
      | }
      | ^

Solve that by unifying the code base on g_assert_not_reached() instead.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240910221606.1817478-5-pierrick.bouvier@linaro.org>
[PMD: Add description suggested by Eric Blake]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>