Security: fix redis-cli buffer overflow.

Thanks to Fakhri Zulkifli for reporting it.

The fix switched to dynamic allocation, copying the final prompt in the
static buffer only at the end.
static char *hintsCallback(const char *buf, int *color, int *bold) {
    if (!pref.hints) return NULL;

    int i, argc, buflen = strlen(buf);
    sds *argv = sdssplitargs(buf,&argc);
    int endspace = buflen && isspace(buf[buflen-1]);

    /* Check if the argument list is empty and return ASAP. */
    if (argc == 0) {
        sdsfreesplitres(argv,argc);
        return NULL;
    }

    for (i = 0; i < helpEntriesLen; i++) {
        if (!(helpEntries[i].type & CLI_HELP_COMMAND)) continue;

        if (strcasecmp(argv[0],helpEntries[i].full) == 0)
        {
            *color = 90;
            *bold = 0;
            sds hint = sdsnew(helpEntries[i].org->params);

            /* Remove arguments from the returned hint to show only the
             * ones the user did not yet typed. */
            int toremove = argc-1;
            while(toremove > 0 && sdslen(hint)) {
                if (hint[0] == '[') break;
                if (hint[0] == ' ') toremove--;
                sdsrange(hint,1,-1);
            }

            /* Add an initial space if needed. */
            if (!endspace) {
                sds newhint = sdsnewlen(" ",1);
                newhint = sdscatsds(newhint,hint);
                sdsfree(hint);
                hint = newhint;
            }

            sdsfreesplitres(argv,argc);
            return hint;
        }
    }
    sdsfreesplitres(argv,argc);
    return NULL;
}

static char *hintsCallback(const char *buf, int *color, int *bold) {
    if (!pref.hints) return NULL;

    int i, argc, buflen = strlen(buf);
    sds *argv = sdssplitargs(buf,&argc);
    int endspace = buflen && isspace(buf[buflen-1]);

    /* Check if the argument list is empty and return ASAP. */
    if (argc == 0) {
        sdsfreesplitres(argv,argc);
        return NULL;
    }

    for (i = 0; i < helpEntriesLen; i++) {
        if (!(helpEntries[i].type & CLI_HELP_COMMAND)) continue;

        if (strcasecmp(argv[0],helpEntries[i].full) == 0)
        {
            *color = 90;
            *bold = 0;
            sds hint = sdsnew(helpEntries[i].org->params);

            /* Remove arguments from the returned hint to show only the
             * ones the user did not yet typed. */
            int toremove = argc-1;
            while(toremove > 0 && sdslen(hint)) {
                if (hint[0] == '[') break;
                if (hint[0] == ' ') toremove--;
                sdsrange(hint,1,-1);
            }

            /* Add an initial space if needed. */
            if (!endspace) {
                sds newhint = sdsnewlen(" ",1);
                newhint = sdscatsds(newhint,hint);
                sdsfree(hint);
                hint = newhint;
            }

            sdsfreesplitres(argv,argc);
            return hint;
        }
    }
    sdsfreesplitres(argv,argc);
    return NULL;
}
