mmap-wx: always put a final \0 to make valgrind happy

vlagrind was complaining about the buffer used to receive the target symlink
path not to be initialised:

    Conditional jump or move depends on uninitialised value(s)
       at 0x4E7B1F0: vfprintf (in /usr/lib/libc-2.19.so)
       by 0x4E7E630: buffered_vfprintf (in /usr/lib/libc-2.19.so)
       by 0x4E796BD: vfprintf (in /usr/lib/libc-2.19.so)
       by 0x4F26BC0: __printf_chk (in /usr/lib/libc-2.19.so)
       by 0x109131: test_mmap_wx_dir (in /tmp/mmap-wx)
       by 0x108DC5: main (in /tmp/mmap-wx)

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>
mmap-wx: minor changes to make output nicer

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>