Fix buffer overflow

`sprintf()` writes terminate `'\0'` at the end. So `sprintf(...,
"%016d", ...)` needs 17 (16 + 1) bytes for buffer.

We can detect buffer overflow by adding `-D_FORTIFY_SOURCE=2` to
compile flag like:

    diff --git a/Makefile b/Makefile
    index a8ae9ba..059d1d4 100644
    --- a/Makefile
    +++ b/Makefile
    @@ -1,5 +1,5 @@
     CC = gcc
    -CFLAGS = -Wall -O2 -lgroonga -I/usr/include/groonga -ltokyocabinet
    +CFLAGS = -Wall -O2 -lgroonga -I/usr/include/groonga -ltokyocabinet -D_FORTIFY_SOURCE=2

     all: grn_hash_bench tchdb_bench