Fix nasa#499, shmat() in cfe_psp_memory.c uses NULL instead of zero.

[myc-yang]

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Describe the contribution
Fixes #499

Expected behavior changes
No Impact on Behavior

Testing performed
GitHub Actions and Axle

Expected behavior changes
Resolve Axle warnings on uninitialized variables

System(s) tested on
GitHub Actions and Axle.

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Michael Yang / NASA GSFC

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[jphickey]

To be pedantic -- this should also remove the (void*) cast.

NULL is already a pointer-type value. It should not be cast as such.

[myc-yang]

@jphickey - Thanks, I made the change.