Fixes for GCC 4.7.0

Fixes for GCC 4.7.0

Fixes the following compile error exposed with GCC 4.7.0:

    libarchive/archive_string.c: In function 'cesu8_to_unicode':
    libarchive/archive_string.c:2450:11: error: 'wc' may be used uninitialized in this function [-Werror=uninitialized]
    cc1: all warnings being treated as errors

As well as a test failure that depends on signed integer wraparound,
which is a very bad thing to do in C [1]. Mark the intermediate result
as volatile to prevent the compiler optimizing away the arithmetic and
the logical test.

[1] http://www.gnu.org/software/autoconf/manual/autoconf-2.67/html_node/Signed-Overflow-Examples.html