MdePkg/UefiDevicePathLib: Add sanity check for FilePath device path

MdePkg/UefiDevicePathLib: Add sanity check for FilePath device path

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1497

Current implementation of IsDevicePathValid() is not enough for type
of MEDIA_FILEPATH_DP, which has NULL-terminated string in the device
path. This patch add a simple NULL character check at Length position.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>