MdeModulePkg: DxeCore: Fix Use-After-Free guard causing page fault

[kuqin12]

Description

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2411

With Use-After-Free heap guard feature enabled, the DxeCore would blindly attempt to "level-up" when the GuardAllFreedPages inspect a non-max level table entry from the last loop. This could cause the next round of inspection to dereference a potentially null pointer and as such causing a page fault.

This change adds a null pointer check to prevent such case from happening.

• [ ] Breaking change?
  • Breaking change - Does this PR cause a break in build or boot behavior?
  • Examples: Does it add a new library class or move a module to a different repo.
• [ ] Impacts security?
  • Security - Does this PR have a direct security impact?
  • Examples: Crypto algorithm change or buffer overflow fix.
• [ ] Includes tests?
  • Tests - Does this PR include any explicit test code?
  • Examples: Unit tests or integration tests.

How This Was Tested

This is tested on QEMU based virtual platforms.

Integration Instructions

N/A

[kuqin12]

@lgao4 Could you please review the PR and provide feedback when you have a chance? Thanks!

[Javagedes]

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[kuqin12]

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[kuqin12]

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