In this patch, we replace all zero characters to '@'.

==30==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60300000e3ca at pc 0x7f34144b6be1 bp 0x7ffee7b6bb20 sp 0x7ffee7b6b298
READ of size 26 at 0x60300000e3ca thread T0
    #0 0x7f34144b6be0  (/lib64/libasan.so.3+0x8dbe0)
    #1 0x7f34144b8e4d in __interceptor_vsnprintf (/lib64/libasan.so.3+0x8fe4d)
    #2 0x4966cb in vprint_on_level criu/log.c:228
    #3 0x496b64 in print_on_level criu/log.c:249
    #4 0x505c94 in collect_one_unixsk criu/sk-unix.c:1401
    #5 0x4e7ae3 in collect_image criu/protobuf.c:213
    #6 0x462c5c in root_prepare_shared criu/cr-restore.c:247
    #7 0x462c5c in restore_task_with_children criu/cr-restore.c:1420
    #8 0x7f34132d70ec in __clone (/lib64/libc.so.6+0x1030ec)

0x60300000e3ca is located 0 bytes to the right of 26-byte region [0x60300000e3b0,0x60300000e3ca)
allocated by thread T0 here:
    #0 0x7f34144efe70 in malloc (/lib64/libasan.so.3+0xc6e70)
    #1 0x7f3413bdb021  (/lib64/libprotobuf-c.so.1+0x6021)
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>