MdePkg/PiFirmwareFile: fix undefined behavior in SECTION_SIZE

MdePkg/PiFirmwareFile: fix undefined behavior in SECTION_SIZE

RH covscan justifiedly reports that accessing
"EFI_COMMON_SECTION_HEADER.Size", which is of type UINT8[3], through a
(UINT32*), is undefined behavior:

> Error: OVERRUN (CWE-119):
> edk2-89910a39dcfd/OvmfPkg/Sec/SecMain.c:178: overrun-local: Overrunning
> array of 3 bytes at byte offset 3 by dereferencing pointer
> "(UINT32 *)((EFI_COMMON_SECTION_HEADER *)(UINTN)Section)->Size".
> #  176|       Section = (EFI_COMMON_SECTION_HEADER*)(UINTN) CurrentAddress;
> #  177|
> #  178|->     Size = SECTION_SIZE (Section);
> #  179|       if (Size < sizeof (*Section)) {
> #  180|         return EFI_VOLUME_CORRUPTED;

Fix this 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
Issue: scan-1007.txt
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>