MdePkg/PiFirmwareFile: fix undefined behavior in FFS_FILE_SIZE

MdePkg/PiFirmwareFile: fix undefined behavior in FFS_FILE_SIZE

Accessing "EFI_FFS_FILE_HEADER.Size", which is of type UINT8[3], through a
(UINT32*), is undefined behavior. Fix it by accessing the array elements
individually.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=1710
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>