RTX5: CVE-2021-27431 vulnerability mitigation

Reported vulnerability:
integer wrap-around in osRtxMemoryAlloc function (parameter 'size')

osRtxMemoryAlloc is an internal RTX function and is not intended to be
called directly by the user. It is also not exposed as public API.

Internal usage of osRtxMemoryAlloc ensures that ‘size’ is never
a large value (>=0xFFFFFFF8U) that would lead to wrap-around.