string_test: we should always use 'unsigned int' for b

string_test: we should always use 'unsigned int' for b

As recently shown in fb0ac2 (s/int/unsigned/ since GCC 4.3.2 produces
code which doesn't stop)

comparing (signed) int variable to 0x80000000 is not idea for x86_64.

This is not a good idea for x86_32 either, because GCC 4.3.2 (the one in
Debian Lenny) rightly assumes that a signed int could be never
0x80000000, and thus removes the check from

    while (b != 0x80000000) {
        ...

completely.

If we want this check, we need b to be always 'unsigned'

Signed-off-by: Kirill Smelkov <kirr@landau.phys.spbu.ru>