zdtm/chroot: allocate enough memory for a path

==36==ERROR: AddressSanitizer: heap-buffer-overflow on address
	0x60200000001c at pc 0x7fb26c88d5f9 bp 0x7ffc15087d40 sp 0x7ffc150874d0
WRITE of size 13 at 0x60200000001c thread T0
    #0 0x7fb26c88d5f8 in vsprintf (/lib64/libasan.so.4+0x9e5f8)
    #1 0x7fb26c88d986 in __interceptor_sprintf (/lib64/libasan.so.4+0x9e986)
    #2 0x402453 in main /root/git/main/criu/test/zdtm/static/chroot.c:68
    #3 0x7fb26c43e4d9 in __libc_start_main (/lib64/libc.so.6+0x204d9)
    #4 0x4031b9 in _start (/root/git/main/criu/test/zdtm/static/chroot+0x4031b9)
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm/static/chroot.c

@@ -64,7 +64,7 @@ int main(int argc, char **argv)
 
 	test_init(argc, argv);
 
-	filepath = malloc(strlen(filename) + 1);
+	filepath = malloc(strlen(filename) + 2);
 	sprintf(filepath, "/%s", filename);
 
 	pipe(pipe_prep);