[CHERRY-PICK] NetworkPkg/SnpDxe: Fix Snp used uninitialized

[eeshanl]

Description

Ensures the Snp Structure is initialized as NULL.

cherry-picks back the fix from EDK2

For details on how to complete these options and their meaning refer to CONTRIBUTING.md.

• Impacts functionality?
• Impacts security?
• Breaking change?
• Includes tests?
• Includes documentation?

How This Was Tested

Tested on physical device

Integration Instructions

N/A

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/202502@ee61221). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
NetworkPkg/SnpDxe/Snp.c	0.00%	2 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##             release/202502   #1554   +/-   ##
================================================
  Coverage                  ?   0.55%           
================================================
  Files                     ?     165           
  Lines                     ?   71948           
  Branches                  ?    1750           
================================================
  Hits                      ?     402           
  Misses                    ?   71539           
  Partials                  ?       7           

Flag	Coverage Δ
NetworkPkg	0.55% <0.00%> (?)

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[os-d]

Please add [CHERRY-PICK] to the commit title

[eeshanl]

Please add [CHERRY-PICK] to the commit title

done