[CHERRY-PICK] MdeModulePkg/DxeCapsuleLibFmp: Check for NULL in IsValidCapsuleHeader

[zurcher]

Description

Add a NULL check in IsValidCapsuleHeader before dereferencing CapsuleHeader

• Impacts functionality?
• Impacts security?
• Breaking change?
• Includes tests?
• Includes documentation?
• Backport to release branch?

How This Was Tested

Published HOB with EFI_HOB_TYPE_UEFI_CAPSULE and BaseAddress = 0.
Confirmed IsValidCapsuleHeader returned FALSE instead of crashing.

Integration Instructions

N/A

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Please upload report for BASE (dev/202502@3ecdf7e). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...ModulePkg/Library/DxeCapsuleLibFmp/DxeCapsuleLib.c	0.00%	4 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             dev/202502    #1290   +/-   ##
=============================================
  Coverage              ?    0.61%           
=============================================
  Files                 ?      613           
  Lines                 ?   219906           
  Branches              ?        0           
=============================================
  Hits                  ?     1349           
  Misses                ?   218557           
  Partials              ?        0           

Flag	Coverage Δ
MdeModulePkg	0.61% <0.00%> (?)

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[zurcher]

Build failure is not in my change:

ERROR - Compiler #error from /__w/1/s/UnitTestFrameworkPkg/Library/Posix/MemoryAllocationLibPosix/MemoryAllocationLibPosix.c variable ‘Length’ set but not used [-Werror=unused-but-set-variable]