MdeModulePkg:Fix bug in function AsciiStrToIp4.

MdeModulePkg:Fix bug in function AsciiStrToIp4.

If a FQDN contains 3 dots '.' like "a.b.c.com", the AsciiStrToIp4
will return success as the HostName has a valid IP address. So we
need to check if it is a decimal character before using AsciiStrDecimalToUintn.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Sriram Subramanian <sriram-s@hpe.com>