initialize null terminator of new printbuf

[ploxiln]

It's possible (e.g. by using json_object_from_file() on an empty file)
to get json-c to try to use a printbuf that has never had anything
written to it. Before this change, it could access a string that
should be length zero, but was never initialized, and could
theoretically have an unexpected string.

I ran into this while using valgrind on an application's tests that sometimes
specified an empty json configuration file. So I guess the main motivation of
this PR is to shut up valgrind :)

But, I was able to come up with a sort of proof-of-concept of actual harm from the bug:

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include "json.h"
#include "json_util.h"

const char *test_file = "test_attack.json";

int main() {
    int fd = open(test_file, O_CREAT | O_TRUNC);
    if (fd < 0) {
        perror("failed to open test file");
        return 1;
    }
    close(fd);

    int i;
    char *bufs[4];
    for (i = 0; i < 4; i++) {
        bufs[i] = malloc(32);
        strcpy(bufs[i], "{\"evil\": true}");
    }
    for (i = 0; i < 3; i++) {
        free(bufs[i]);
    }

    json_object *jso = json_object_from_file(test_file);
    if (jso != NULL) {
        fprintf(stderr, "! json from empty file ! %s\n", json_object_get_string(jso));
        return 1;
    }
    return 0;
}

$ ./test_attack 
! json from empty file ! { "evil": true }

EDIT: tweaked example code loop counts to make the trick work on more systems