Fix warning strncpy bound equals destination size

Fix warning strncpy bound equals destination size

- Reduce num from 5 to 4 in strncpy and add null terminator.
  Remark: Version strings consist of 4 chars, e.g. RRD_VERSION5 "0005"

- Fixes the following GCC 8 warnings:
  rrd_create.c:960:9: warning:
    ‘strncpy’ specified bound 5 equals destination size
    [-Wstringop-truncation]
    strncpy(rrd.stat_head->version, require_version, 5);

  rrd_modify.c:1218:9: warning:
    ‘strncpy’ specified bound 5 equals destination size
    [-Wstringop-truncation]
    strncpy(out->stat_head->version, require_version, 5);