Fix nasa/sc#176, Replaced zero with NULL in #define SC_MEMBER_SIZE.

[myc-yang]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to GSFC-SoftwareRelease@mail.nasa.gov and copied cfs-program@lists.nasa.gov.

Describe the contribution
Fixes #176 - Replaced zero with NULL in #define SC_MEMBER_SIZE.

Testing performed
Ran the Axle static code analysis tool and it no longer reported any findings in sc_ats1.c, sc_rts001.c, sc_rts002.c, sc_rts003.c, and sc_rts004.c when I ran the literal-zero-used-as-null-pointer query.

Expected behavior changes
No impact to behavior

System(s) tested on

• Hardware: PC
• OS: Linux 9.8

Contributor Info - All information REQUIRED for consideration of pull request
Michael Yang / NASA GSFC