Fix #418, malloc stub memalign calculation

[jphickey]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
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Describe the contribution
Correct the round-up memory alignment calculation in the stub "malloc" routine.

Fixes #418

Testing performed
Run all unit tests

Expected behavior changes
malloc stub now rounds up properly

System(s) tested on

• Hardware: [e.g. PC, SP0, MCP750]
• OS: [e.g. Ubuntu 18.04, RTEMS 4.11, VxWorks 6.9]
• Versions: [e.g. cFE 6.6, OSAL 4.2, PSP 1.3 for mcp750, any related apps or tools]

Additional context
Same change as was submitted in nasa/osal#1413

It is likely this was never really used by tests, as this bug had been in there a long time and not noticed. At least with this it should work as intended if multiple blocks are allocated.

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