Fix #123, use CFE_MSG_PTR conversion macro

[jphickey]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
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Describe the contribution
Rather than taking the address of a sub-member, use the macro provided from the CFE_MSG module to convert from a local container to a CFE_MSG_Messge_t pointer.

Fixes #123

Testing performed
Build and run all tests

Expected behavior changes
None.

System(s) tested on
Debian

Additional context
This makes the source code more compatible with alternative implementations of the CFE_MSG module (e.g. if sub-structs are not named identically).

Contributor Info - All information REQUIRED for consideration of pull request
Joseph Hickey, Vantage Systems, Inc.

[jphickey]

Superseded by #125 (dependency issue)